The little holding tank sensor. Oh, how we loathe your very existence. Let’s take a look at modern man’s hated RV enemy and try to understand them a bit better.

Your RV’s holding tanks

As you likely already know, your RV has three types of holding tanks. First is the fresh water tank that holds a supply of potable water you can use for drinking, cooking, cleaning, bathing, and is used when your toilet is flushed.

Next is the gray water tank that contains anything that goes down your galley sink drain, shower or tub drain, and, in most cases, your bathroom or vanity sink drains (some RV, like our Airstream, for instance, drain the bathroom sink into the black tank).

Finally, you have the black tank, where anything put into the toilet goes. In some cases — mostly smaller RVs — the black and gray tanks may be combined into one tank.

Tank sensors or the dumbest things ever invented by man

Right? Hey, we’re right there with you. It seems something so simple — a method to measure and report the relative fullness of a holding tank — just seems absolutely impossible to design and manufacture with any amount of reliability or credibility. We hear it all the time. Like every. Single. Job. We. Do.

In most cases, the sensors in your tanks are an array of little metal, electric-conducting sensors or nubs that stick into the tank wall from the outside. There’s usually four or five of them: One, on one side (alone), that sends a current into the tank’s contents. On the other side of the tank are three or even four more evenly spaced up the side the tank.

The idea is, a current is sent through the one sensor on one side and, as the level of liquid rises in the tank, sensors on the other side are submerged and the electrical current travel through the liquid and is picked up the appropriate sensors on the other side. The higher the water or liquid (indicating an increasing level of fullness), the higher up on the wall a sensor is activated to complete the electric circuit.

This electric circuit is transmitted to the reporting panel which (should) indicate how full the tank is. At least that’s the theory.

Note that some newer RVs use the SeeLevel tank sensing system which acts a bit differently. We note that this system is entirely always accurate either which indicates that mankind is still working on the problem!

What makes my tank sensors inaccurate?

Simply put, it’s the crap, crud, and buildup in your tanks. “But I use my onboard black tank spray system religiously.” Great, you’re doing better than 50% of the RVers out there. But that spray system sprays in a set pattern (that may or may not be where your sensors are located) and only at 100-150 PSI (which is about two to three times the pressure of “shore” or RV park-provided fresh water. Plus, those spray system nozzles get clogged over time (you do clean them annually, don’t you?!).

Most inaccurate reading from tank sensors happen because human waste and toilet paper (in the case of black tanks), food particles (in the case of gray tanks), or other stuff put down the tanks sticks to the walls of the holding tanks, preventing the sensors from functioning properly.

Think about it. Your holding tanks are a “wet” or moist environment. They probably rarely dry all the way and there is likely some sticky, gooey, or otherwise “thick” stuff in there, adhering to the walls, top, and anything else (like the sensor nubs). It’s not exactly an environment that is conducive (!) to electrical measurement.

But what about tank additives, enzymes, etc.?

These are designed to digest solids in your tanks (especially the black one, but also the gray tank) and turn it into something a bit more liquid. The problem? That liquid is thick. And sticky. And gets stuck on…you guessed it, the sensors. And it’s sometimes hard to get off.

So using an enzyme or dissolving chemical is actually making the situation worse. So stop using them.

What’s the solution?

First, you need to get your tanks super clean. Like they were when they were brand new, from the factory. There’s only one way to do that. That’s to use a tank cleaning professional like Kleen Tank or one of our All Pro Water Flow sister companies. We use high-pressure water (1500-3000 PSI) to scrub the surfaces of your holding tanks — from the inside. EVERYTHING gets scrubbed clean, including those sensors.

Then, ongoing, here’s what you can do to maintain that clean environment in your holding tanks once they have been professionally scrubbed clean:

  1. Stop using enzymes and chemicals. Otherwise, you’re just starting the cycle of stickiness again.
  2. Empty your tanks frequently. Every two to three days is optimal and make sure to top off each tank so it is full before you empty it.
  3. Use that onboard tank rinsing system. It’s better than nothing and keeps things moving when you are emptying the tanks.
  4. Backflush each of your tanks every other emptying. Use something like the
    Valterra Flush King Backflush Valve
    to truly rinse out each tank and keep it clean.
  5. Get your tanks professionally cleaned once a year (if you’re a some- or part-timer) or more often (if you are a full-timer, have an older RV, or have persistent tank problems).