An RV or a motorhome with a bathroom or kitchen facility typically has two tanks. One that stores freshwater, and one that stores black or “Gray Water.” Each of these tanks has its own special sensor or meter to let you know how much liquid there is inside.

When your RV’s black water tank sensor is damaged or working improperly it can certainly be frustrating. If the tanks are nearly full, it can even be worrisome!

So how can you know for sure how much black water there is in the tank? And how to clean RV black water tank sensors to address a faulty sensor?

When it comes to a corroded or dirty sensor, you might be able to clean it yourself. However, there can be some complications along the way of cleaning RV holding tank sensors.

What’s In A Black Water Tank?

Gray water or black water can have a lot of things in it that can gum up the sensor or cause malfunctions.

This tank essentially contains all the used water that goes down the drain. It includes:

  • Shower and bathtub water
  • Kitchen and bathroom sink water
  • Floor drain
  • Dishwasher rinse water
  • Washing machine rinse water

This means that the so-called “Gray Water” in the tank can have a lot of soap, grease, body soil, and food particles.

Any of these substances can leave hard to remove residue on the sides of the tank and on the sensor itself.

If the problem is bad enough, it could cause the black water tank sensor to fail or send a misreading.

Periodically purging your black water tank and cleaning it will help reduce chances of a sensor malfunction.

Many RV parks will have these facilities available. If you are new to your RV, you might want to seek out someone with more experience, or perhaps a professional service that can shed some light on how to perform proper maintenance.

How Does A Black Water Tank Sensor Work?

The sensor inside a black water tank looks like little more than small metal nubs that are attached to the sides of the tank with small screws or fasteners.

How Does A Black Water Tank Sensor Work

The design is simplistic. When the fluid level rises high enough the water conducts and small charge, which essentially completes the circuit, and trips the colored lights on the indicator panel.

Most RV manufacturers place multiple sensors at different heights. They turn on as the tank fills to let you know when it’s at a third, half, three-quarters, or nearly full. 

Why Black Water Tank Sensor Not Working?

Sometimes wet physical debris can get caught on the sensors inside the tank, which completes the circuit, and ultimately causing a false reading.

This is even more likely to happen if you are going up or down a steep grade, or perhaps taking a sharp turn.

If you notice a light turn on when you’re going up or downhill, and it stays on, you might have some wet debris gumming up some of the sensors.

This is even more likely to be an issue if you notice something like a half-full sensor turn on, but the sensor below it does not.

Toilet Paper Can Be A Problem

There are some people who are very particular about the brand of toilet paper they use in their home. However, it’s hard to carry these personal preferences into your RV bathroom.

You see when you flush your toilet at home, the material and the toilet paper moves through the lines to the municipal sewer or a rural septic tank.

The toilet paper then gradually breaks down over time. In an RV black water tank, ordinary toilet paper can cling to the sensors, cause clogs and other complications.

To reduce the chances of this happening, you need to buy toilet paper that is specifically rated for RVs.

Manufacturers specifically design the fibrous blend to break down quickly to prevent the problems caused by ordinary toilet paper. Particularly, getting caught on the tank sensors.

Cleaning Your RV Black Water Tank In 3 Easy Steps

Flushing and purging with clean, freshwater is more of a maintenance procedure. If you have an older RV, or the tank has gone a little too long without a good purge, you should really consider giving it a deep cleaning.

There are a few different ways you can do this. The one that’s right for you is a little bit of a matter of personal preference.

Option Number One: Dishwashing Detergent With Ice Cubes

This is a convenient option if you are on the go and need to give your black water tank a thorough cleaning. In this method, you don’t want to use liquid soap, as it will cause foam and bubbles.

  • Step One: Make sure the tank valve is tightly closed.
  • Step Two: Fill the tank half full with fresh water.
  • Step Three: Add half a cup of granulated dish-washing detergent. (Do not use liquid soap)
  • Step Four: Go for a casual drive of half an hour or more.
  • Step Five: Let the solution sit overnight so any bubbles settle out and the soap break down any greasy deposits.
  • Step Six: The next morning dump the black water tank and give it a quick freshwater purge.

Option Number Two: Use A Tank Cleaning Wand

This is probably the most straight and direct way to clean your RV’s black water tank. You need to buy a special attachment, which costs less than $20 online.

The wand attachment is specifically designed to spray water with relative pressure from side to side.

Use A Tank Cleaning Wand (1)
  • Step One: Attach the cleaning want to the end of a garden hose
  • Step Two: Run it through a window.
  • Step three: Feed the attachment and the host through the toilet to the black water tank. This can be a little awkward.
  • Step Four: Once it feels like the wand has reached the blackwater tank, have someone turn on the hose.
  • Step Five: Open the release valve on the back of the black water tank.
  • Step Six: Gently move the hose back and forth letting it run until the water coming out of the back runs clear.

When you pull the wand out of the toilet, remember to wear gloves, eye protection, and keep your mouth closed.

It has a knack for spraying out little drips of water right after being in a bacteria-laden tank.

Option Number Three: Use Water Mixed With Chlorine Bleach

This option is a little controversial, in that heavy concentrations of bleach are known to damage certain rubber plumbing parts.

So, it’s not something you should be cavalier with. Yet chlorine does a great job of disinfecting the black water tank, killing bacteria and leaving behind a pleasant clean odor.

  • Step One: Run a garden hose through a window to the bathroom or a large drain.
  • Step Two: Fill the black water tank to roughly half full with fresh water.
  • Step Three: Pour half a cup of quality bleach down the drain into the blackwater tank.
  • Step Four: Continue adding fresh water until the tank is nearly full. Then wait five to seven minutes.
  • Step Five: Release the rear valve on the black water tank and all it to drain completely.
  • Step Six: Seal the rear valve, and refill the tank, then purge it again.

This will eliminate any remaining bleach in the tank after its done killing the germs and freshing the odors.

How Often Do I Need To Flush My Black Water Tank?

Sometimes even the most well-maintained black water tank, with RV toilet paper in it will still have a sensor malfunction.

The best way to prevent these frustrating faulty sensor readings is to routinely flush your black water tank and to use a good tank treatment.

There are some people who like to flush their RV’s black water tanks almost every time they dump.

How Do I Flush My RV’s Black Water Tank?

Flushing your RV’s black water tank is relatively easy. You can usually do this at an RV park that has water hookups and dump services.

How Do I Flush My RV’s Black Water Tank?
  • Step One: Drain the gray water out of your black water tank.
  • Step Two: Fill the tank with freshwater until the tank sensor reads Full.
  • Step Three: Drain the water out of the tank.

If the water doesn’t drain clear, or the tank sensor isn’t giving you an accurate reading, you may need to refill the tank and dump it again.

There was one time when my friend and I drained and flushed his RV tank three times, and the sensor was still acting like there was something stuck on it.

In a moment of desperate improvisation, we filled the tank to what we assumed was around halfway, then we drove around the back roads surrounding Cheyenne, Wyoming for half an hour. It ended up working, but it wasn’t perhaps the best idea.

In this case, the tank had been poorly maintained. If you are flushing your black water tank after every other drain, it will greatly reduce your chances of having something get stuck on the sensors.

How Do I Fill My Black Water Tank With Fresh Water?

This is certainly easier at an RV park or at home where you have freshwater hookups regularly available.

You simply let the toilet run or dump buckets of water down the drain. If you are at home, a simple garden hose poured down the shower drain will fill it in a hurry.

How Do I Flush My RV’s Black Water Tank

A more convenient option is to buy a special sewer elbow with a water hose attachment.

This allows you to full the black water tank from outside, without having to pass a garden hose through the window.

Just keep a dedicated garbage bag, or an old plastic comforter case to store the sewer elbow when you take it out.

The thing has a real knack of sticking little streaks of somewhat foul residue on it from time to time.

In Conclusion

There’s nothing like routine maintenance to help prevent problems when you are on the road.

Taking the time to clean out your black water tank before you leave on a trip reduces the chances of settled sludge from causing a problem.

When you are at an RV park on your adventure, taking the extra time to flush out your black water tank after every dump/drain, will go a long way toward helping things flow smoothly.