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Your RV Holding Tank Sensors Explained

Kelly Headshot

By Kelly Beasley

As you probably already know, most modern-day RVs come equipped with 2-3 camper holding tanks.

One holds fresh water, another holds sink and shower water, and one more that holds toilet waste.

These are called the RV gray water tank (sink/shower), the camper black water tank (toilet), and the fresh tank (clean water).

Kelly and Marshall's RVs at Sauls Creek in Bayfield, Colorado

There are RV holding tank level sensors that keep track of how full or empty any given tank is at any given time... or at least that is what the sensors are SUPPOSED to do.

However, more often than not, one or two of your RV waste tank sensors will mis-read due to obstructions inside the tank.

This is super common, so if you are experiencing RV tank level indicator sensor malfunction, don't stress it.

Here you will learn all about your RV holding tank sensors, how they work, and how to correct sensor issues.

Why Do I Need RV Tank Sensors?

You only need (need is a strong word) RV tank sensors when you dry camp.

Contrarily, if you camp ONLY in campgrounds with full hook-ups, you do not need tank sensors. Why? Because you'll always be connected to utilities so you don't use the tanks.

The job of a holding tank level sensor is reading how empty or full the tanks are. When you're connected to water and sewer, there's no need to fill any of your tanks for long or at all.

You benefit more from holding tank sensors if you camp without utility connections.

Because now you are self-contained. You store your wastewater inside your waste tanks. They are only so large.

Therefore, you want to know when they are getting close to empty (freshwater) or full (gray and black) so you don't run out of water or overflow your toilet or sinks/shower drain.

Let's dive a little deeper into why you don't need to use your waste tank sensors when you are in a campground with full hook-ups.

Fresh Water Tank:

Instead of filling your freshwater tank with water, you'd connect your water hose to the city water infill.

RV city water hookup

An outside water hose attachment on an RV.

Then, the water coming into your RV goes directly into the pipes, bypassing that holding tank.

Therefore, there's no filling of the tank and no need to know how much is in there.

Black Tank:

You also don't NEED your black tank sensors to tell you when your toilet tank is full.

You can either dump your black tank once a week or so, or you will 'see' or hear when it's time.

How will you see or hear it? Because your toilet either won't flush anymore, or it will 'burp' before that time comes, and boom, you know it's time to dump.

Gray Tank:

When it comes to the gray tank, if you are connected to a sewer, your can keep your gray valve open so it never fills up.

Hence, there is no need for tank sensors to tell you when you must find a dump station to go dump into.

Technically, if you ALWAYS camp with full hook-ups, you don't need to use your installed waste tank sensors.

Holding tank

When Do You Need To Use RV Holding Tank Sensors?

RV tank sensors are helpful when you are camping without full hook-ups, meaning when you are dry camping.

When camping like this, there's no water or sewer connection.

Therefore, your freshwater tanks hold the water you need for washing dishes, showering and that you rinse stuff with.

Your gray tank collects and holds that waste water. Your black water holding tank collects and holds the toilet water.

Now your sensors are handy to tell you when they are getting (freshwater) close to empty or getting close to full. When that time comes, it's time to dump and fill.

Two Different Sensor Types

There are two different types of holding tank sensors: probe style and capacitive style.

Probe-style sensors are by far the most commonly used in recreational vehicles. This is because they are cheap, and most manufacturers want to put the least expensive components into an RV as possible.

Tank probe

Probe-style sensor

Tank monitor panel

Tank levels showing in thirds

These probe-style sensors protrude into the tank, passing through the tank wall. The interior portion of the sensor is what actually 'reads' the tank levels.

An electrical wire that connects to the RV tank level monitoring system will be on the outside of the probe.

Because of the way this type of sensor works, and because there are only 3-4 sensor positions in total, these systems aren't capable of giving a precise level reading. They are also susceptible to giving false readings due to contamination (as we will discuss in the next section).

A more accurate and more expensive monitoring system uses capacitive sensors mounted on the outside of the tank. There is no intrusion into the tank like probe-style sensors, so they don't suffer from the false reading issue.

SeeLevel tank monitor

Monitor showing levels in percent

These capacitive sensors also give far more accurate readings of the tank levels, displaying the levels in a percentage. They will show an accuracy of around 8%, which is a lot better than a probe sensor setup.

You will typically find this style of a sensor on upper-end recreational vehicles.

How Do RV Holding Tank Sensors Work?

The most common RV holding tank sensors work by checking if a simple electrical circuit is open (not making contact) or closed (making contact).

RV tank level sensors form a low-wattage open circuit. As long as there is no liquid or something else completing the circuit from the inside, it stays open.

Along comes liquid levels above each sensor, and that liquid completes that circuit.

So, if the fluid in the tank is up to the halfway mark, the half mark sensor closes, showing it as half full.

Each RV holding tank level indicator sensor has a wire that runs from the sensor to the RV tank level monitor panel. There, it displays the level inside each tank.

Family sitting in chairs in front of Class C motorhome

Great as that sounds, it's a flawed design, as things other than liquids can get caught on the other side of the sensor, causing a misread.

This includes toilet paper, struvites, food particles, grease, etc.

The issue is not necessarily a sensor problem. Instead, the RV tank sensors may work just fine.

It's that there is debris caught inside the tank causing the monitor to display a false reading.

To take proper care, be sure to use toilet paper safe for RV use (better yet, don't flush your paper at all), and try to keep as much dishwashing waste (food debris and grease) from going down your drain.

There is a style of RV water tank sensor that mounts to the outside of the tank and uses electronics to measure the capacitive difference in liquid levels inside the tank.

This system is much more accurate than a traditional probe-style RV water tank level sensor but costs more, so you will typically only see it on higher-end RVs.

However, these exterior mounted RV tank gauge sensors don't collect debris like internally mounted sensors and therefore are practically immune to showing false readings.

What Is A Tank Monitor?

A tank monitor is a small electrical panel in your recreational vehicle that displays the level of your camper's tanks. (It most likely also displays information about other systems and may allow you to control certain appliances.)

The RV holding tank monitor panel receives an electrical signal from the RV water tank capacity sensors and converts that into a visual representation of the tank level.

Tank monitor panel

A basic tank monitor panel

How Do You Read An RV Tank Monitor?

To read your RV tank monitor, first, you must locate yours. Some are displayed out in the open. Others are behind a cabinet door.

The monitor panel has a display that is very simple to read. Some give a percentage full; others give '1/2 full, 3/4 full,' etc.

Some are digital, and others have lights that come on next to a level when you select to see how empty or full a tank is.

RV tank monitors are very simple to read. You most likely will need to press a button to display the reading, and each tank may have its separate button.

How Do You Clean The Sensors On An RV Holding Tank?

Cleaning the sensors on an RV holding tank of goo or debris may be a little tricky. It's likely easiest in your black tank. But we shall cover both the black tank and the gray tank.

(Freshwater tank sensors rarely get contaminated with anything if you only put water into that tank, as directed.)

Read on to find out how to clean RV holding tank sensors.

How Do You Clean Grey Water Tank Sensors

You may need to clean your grey water tank sensors if they get blocked by soap scum, grease, or other residue and debris buildup on the tank walls.

To fix it, the offending stuck residue or buildup causing a reading problem needs a rinse from the walls to clear that sensor.

We believe the first method here to be the most effective solution. But you could try the dishwashing liquid treatment first since it is cheaper and you might have the right soap on hand.

  1. We like the Unique Sensor Cleaner product. It uses bacteria to dissolve all of the solids in a tank. To use, fill your tank to FULL. Add treatment. Leave at least 12 hours for everything to dissolve. Drain. Check sensors. If still reading incorrectly, repeat.
  2. You might try a dishwashing liquid first as a deep clean per the instructions found here. For some, this works as an effective solution.

Liquid RV Tank Sensor Cleaner

The easiest and most effective way to clean RV tank sensors is to use the Unique Sensor Cleaner product. This is especially true when it comes to cleaning RV black tank sensors.

Unique's Sensor Cleaner uses probiotic bacteria and enzymes to dissolve what is causing sensors not to read correctly. They claim it is the most powerful sensor cleaner available.

To use, fill up your tank with water to at least cover the topmost sensor. Shake the bottle of sensor cleaner and pour half of it into the tank.

You don't have to drive your RV. Instead, you just let this solution sit for at least anywhere from 12 to 72 hours (the longer, the better, usually).

Dump and rinse your black tank. Repeat the procedure if necessary.

Unique sensor cleaner

How To Clean RV Black Tank Sensors

The process of RV black water tank sensor cleaning is similar to the grey tank. Use an enzyme or bacterial treatment as mentioned above.

The easiest way to clean black tank sensors is to use the Unique Sensor Cleaner. This liquid enzyme eats away at what is causing the sensors to read incorrectly.

Yes, it can be as easy as that!

Additionally, you can try using a wand with a sprayer to knock off the offending material on the tank walls and sensors. However, this will only work if the problem is caused by loose debris.

If all else fails, there are professional tank cleaners that will give your tanks a deep clean, including the sensors.

Marshall Uses Black Tank Sensor Cleaner

My (Marshall) black tank sensors haven't been reading correctly for years. I'm pretty sure they stopped reading accurately in the first year of ownership.

7 1/2 years later, I finally decided to see if I could get them working again. No time like the present, right?

I used 1/2 a bottle (the usual 'dose') of the Unique Sensor Cleaner product in a full black tank that I filled with fresh water.

After leaving it to sit for four days, I drained the tank, filled it with fresh water to flush it, and drained it again.

(The instructions on the bottle say to let it sit at least 12 hours, while their website suggested 72 hours, saying the longer, the better. So I went with around 96 hours just because I could.)

I observed the black tank level indication after the initial drain, filled it up, and then drained it again.

What do you know? The sensors are working as they should!

The question now is how long it will be before they stop working again? Time will tell.
Overall, this was a super-simple process to get my probe-style sensors reading accurately again.

Try it if you have misbehaving sensors (the treatment won't fix electrical issues, obviously, just 'crud on the sensors' problems). It may do the trick for you as well!

Using An RV Tank Wand To Fix RV Black Water Tank Sensors

There is an RV tank wand or two manufactured to insert into your bathroom toilet.

There, the wand sprays down the interior walls. This works best if your bathroom toilet is installed directly above or near the tank mounted under it.

Find out where your toilet is from your black water tanks. If reasonably close, you can try to clean your black tank sensor using this manual method.

Just keep in mind that this method will only work if the cause of the sensor inaccuracy is loose debris and not the build-up of other material.

Class C motorhome on California Highway 1

There's a product called the Camco Flexible Swivel Stik. This works if your toilet is close to the tank.

The product has a bendable stick, so even if your drain line has a bend in it, the tip can reach down into your tank.

Connect it to a garden hose and let 'er rip! Be VERY CAREFUL that you don't let the tank fill up and overflow.

Best to do this procedure when connected to a ground sewer outlet so you can open the RV waste valve assembly for your black water holding tank.

This way, you cannot overflow the tank into your RV. Allow the sprayer to clean while you lower and lift the device, so you get as much coverage on the walls as possible.

Then, remove, and test your monitor and tank sensor to see if you get a correct indication.

How Do You Know When Your Holding Tank Is Full?

There are a few ways to know when a holding tank is full without using your RV tank sensors.

Black Tanks

Your black tank offers two ways to know you're close to being full sans the holding tank sensor. 

I haven't bothered with seeing how my black tank monitor reads for years now. Instead, I rely on these two things:

  1. You learn how long you can go on a trip before needing to dump. It stays consistent if your habits/number of people using it doesn't change.
  2. Your toilet(s) will announce when they are almost full. Once the sewage level starts to creep up the pipe leading from the toilet down into the tank, it will 'burp' when you flush. You'll notice it. Hard to miss this sign that your tank is very close to being full.

Gray Tanks

Your gray tank also offer ways to know they are full without using your RV holding tank sensors.

  1. Similar to your black tank, you will learn how long you can go on a trip before your tank is nearing full.
  2. Backing up. Your sink and or your shower pan will stop draining. (Usually, your shower shows first, as it's the lowest drain.) This means your tank is full. Bingo! No RV tank sensor is necessary!
Classic car towing teardrop trailer

How Do You Calibrate RV Tank Sensors?

Calibrating RV tank sensors is easy. This has nothing to do with the sensors not working properly.

You don't calibrate when you're getting incorrect readings. You do it when you want to change exactly WHEN the tank shows as almost full or full.

You may decide you need a few days notice before, say, your black tank is actually full.

Then, once it says full, you know you have x number of uses or days until you need to move and dump.

However, most of the lower-end brands have monitors/sensors that are not able to be calibrated.

Higher-end models MAY have monitors that can be calibrated. You know if yours are able to be calibrated if you see small holes on the monitor itself.

Neither mine nor Marshall's RV holding tanks have monitors that can be calibrated. Ours are very common and look like this:

Tank monitor panel

No calibration holes

If yours does have the holes, here's how you calibrate:

Fresh Tank:

This is the easiest one.

  1. Fill your tank with water.
  2. Go inside, push fresh water tank button.
  3. Insert a screwdriver into the hole above the fresh button.
  4. It should read full. If it doesn't, turn to the right until it glows full.
  5. If it shows full, turn it slowly to the left until it turns off.
  6. Then turn it right until it glows red.

Gray Water Holding Tank

Do you really want them to have ZERO room to spare when they read full?

If you want a little bit of wiggle room, keep this in mind when calibrating.

  1. Fill your grey tank until the monitor says it's 3/4 full.
  2. Next, add water down your shower drain until your monitor reads full.
  3. If you want it to show only when it's ABOUT to overflow, fill until you barely see the water creeping up towards the drain.
  4. There, calibrate the monitor to say full.
  5. If you want more wiggle room, once you see the water rising, dial it forward (to the right) until it is 'over' full.

Black Water Tanks

Do this the same way you did the gray tank sensor if you want some wiggle room to know you can still go a day or so once it hits your holding tank level indicator hits the full mark.

Best to do this when connected to a sewer. (But NOT while at a dump station.)

  1. Fill tank until it reads full.
  2. As soon as your black tank says full, start adding water in one-gallon amounts at a time. And count the gallons.
  3. This will let you know how many gallons are truly left once it says full.
  4. Once you can see the liquid when you open the toilet flap, you've reached the number of gallons you had remaining.
  5. If you want more warning time, dial the sensor back a touch. If you wish for less warning time, dial it forward a little.


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RV tank sensors aid RV owners in knowing how full or empty their tanks are.

Though people can live without them, they help keep track of your tanks when you're dry camping.

That said, the gray tank and RV black tank sensors often give incorrect readings at the monitor panel due to blocked sensors.

Sensor issues are pretty easy to resolve by using an RV sensor cleaner.

If you only camp with full hookups, you'll rarely need to use your RV tank monitor system.

But when you do dry camp, you'll be happy you have an RV tank level sensor in every tank!

If you regularly use an RV holding tank sensor cleaner, you're less likely ever to have problems with your RV holding tank monitor system.

If they don't work, don't let that stop you from getting out there and camping! Your RV tank monitor panel and sensors are a luxury, not a necessity!

Kelly Headshot

I dedicated myself to living the full-time RV life for over 6.5 years, immersing myself in the unique quirks and joys of the boondocking lifestyle and gaining a wealth of knowledge and experience along the way. In December 2020, my business partner and I made the transition to part-time RVing, but in January 2023, we hit the road once again, this time in our trusty vans. My mission is to help others embrace the RVing lifestyle with confidence and excitement, armed with the knowledge and resources needed to make the most of their adventures. I believe that the more you know, the more you can truly appreciate and enjoy the freedom and flexibility of the open road.

  • I really appreciate the section on cleaning grey tanks. My black tank has a built-in cleaning system and it works really good. I wish manufacturers would install them on grey tanks too. I’ll try unique on my grey tank that is reading full all the time. This is a 2021 trailer I purchased last week so I don’t know what the previous owner put down the grey tank. I never had this problem on my last trailer

    I disagree with leaving grey tank valves open all the time when hooked up to a campsite sewer. Water enters the tank from the top of the tank and splashes food particles, grease and other contaminants on the sides of the tank, then the water drains out leaving a nasty film on the walls that dry out and create problems with sensors and draining. I let mine fill and then drain. I’m a strong believer that water is your friend when it comes to grey and black tanks.

    • Hi Tom,

      Good points! Yeah, it might be a best practice to keep the grey closed until one dumps. Either way, most people have problems with the sensors working after usually a short time. Most of them/us just give up. We are currently trying a new product (to us) on our new van tanks (We only have grey tanks now).
      We’ll see how it goes. Only been 6 months!

      We hope your black tank sensors continue to work with the system you have in place and we hope your grey gets to working correctly soon.

  • I disagree with your methods and statements of monitors not necessary. I have bad smells when black tank full, no thanks. Also I choose to stay on top of tank levels versus worrying about them. Thanks but I will use monitors.

    • Hi Reva,

      Yes, and to each their own! I found mine to be useless and I simply knew when my tank was getting full per how much I used it, how long it had been, and by seeing/hearing what I saw when I flushed. We’re happy to hear yours work for you, you appreciate them, and you will continue to use them. Cheers!

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