How to tell if your black tank is almost full


For those that are new to RVing…

Factory-installed tank sensors are wholly inaccurate. Previous polls on this site also substantiate this information. Deposits of paper and “other” things get stuck on the sensors. A perfectly empty tank can still read full as a result. Some folks have tried other methods such as dishwashing soap, ice, hot water, and other methods that may work with mixed results.

There is another way to know your tank is full (or close to it) besides using sensors or sound. “Sound” is also a good indicator, but in addition to that method, the closer to full, the more it smells (even if you use chemicals). You’ll want to dump for sure when you can’t take the smell any longer.

“Splash back” is another indicator. Air will get trapped and when you flush, it bubbles back before it goes down.

If the three methods (sensor, sound, smell) fail to warn you of impending doom, you’ll probably get to see a brown liquid coming out at the base of the stool at the floor flange. From there, it will ooze on the floor or into the basement. At that point, you certainly can’t miss it.

There are external sensors that tend to do better than the internal sensors. You can purchase them after-market and use the wires directly from the internally mounted sensors.

We are full-timers. Our black tank is 52 gallons. Without being conservative, we can average about two weeks for two people – sometimes three weeks if we are a little more conservative before we require dumping. Maybe this will help you gauge when a dump may be pending (no pun intended).🙂

Thanks for the tip, TravelingMan!

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Vincent Ruta

folks can do the in out – a simple water meter on your inlet tells you how many gallons of water you took on – simple math if you used 50 gallons of water – it had to go somewhere – so if both your grey tanks are at half – that leaves 10 that has to be in you black tank…

The water meter can sit between your external pressure regulator and the rv water inlet.

Honestly – i try and keep a constant mental note of how much in fluids I am going through.

Ron Graves

Turn the water off and hold the flush open and look down the hole with a flashlight you can see what the level is.


If brown water is leaking out between the toilet base and floor. That is a indication that a new floor flange seal is needed, or the mounting bolts are loose That should be a water tight connection


I only buy an RV that I can look directly into the black water tank – then I can see exactly how much is in it since sensors never work. I made the mistake one time when we bought a 5th wheel whose tank I could not see into – never again. I usually start checking when the gasses escape to inside or water bounces back up when flushing. I can also tell by the sound when I flush how full it is getting.


We have a 20 ft. travel trailer. This is our first RV. We’ve had it. 3+ years now and have put 20,000 miles on it. It has 30 gallon tanks so we need to know waste tank levels to a reasonable accuracy. So I am a relative Rookie at RVing but with 30+ years designing, installing and maintaining measurement equipment. We had problems with false or no readings in each of the tanks after the first year of use. Leaning on my instrumentation experience I knew that this type of sensor was prone to mineral build up on the wet side when used with “hard water”. Of course much of the water we use at campgrounds is well water and/or minimally treated water. Often hard water and certainly not the water quality I enjoy at home. The minerals collect on the sensors when the sensor is energized (whenever I push the button to get a tank level reading). Acetic or other mild acids are regularly used to remove these mineral deposits.

Here’s the method I use annually and I have not had a problem with inaccurate tank readings since:
I add 3 gallons of white vinegar to my fresh tank.
Then I fill the fresh tank to cover the sensors.
This makes a ~30 gallon mix that is a minimum of 10% vinegar.
I let that soak in my fresh tank overnight.
Next day I pump the mixture into my grey tank.
If all the sensors are not covered, I add another gallon of vinegar and more water.
I let that sit overnight.
Next day I pump the mixture from the grey tank to the black tank.
Let the mixture sit in the black tank overnight.
I drain the mixture to the sewer the next day.

WARNING! You must be careful not to let this 10% acetic in water mixture come in contact with your lawn or anything growing that you do not want to kill as this will kill all but the heartiest weeds you have.

This process also disinfects the tanks, pump, water heater and plumbing. While chlorine bleach is much easier to use, an acid disinfectant alternated with bleach disinfectant will keep bleach resistant bacteria (rare but really nasty shtuff) from growing in your plumbing.


When the grey water tanks get dumped the black water tank goes first. You use more grey water than waste deposited. No need to worry about if it’s full or not.


we “solved” the problem by dumping the black tank once a week. issue resolved.


I had the sensor problem on my previous RVs, all purchased used. So when I bought a new RV I decided I would use the black tank flush for EVERY dump, sometimes filling and dumping twice. I also use Pure Power, which works better than anything else I’ve tried. Five years later and a lot of full tanks (we boondock a lot) the sensors are still accurate.

Dale e Rose

I’ve had several RV’s over the last 40 years. and they have all been a straight drop from the toilet into the black tank. Just use a bright flashlight to look down in to see the level.

Donald N Wright

Thank you for the simple solution to a complex problem.

Keith Washburn

Assuming I have the correct charger if I convert the house batteries to lithium ion do I also need to convert my start battery.
Thanks for the info


I keep a square flat flashlight that hangs on a clip above and behind the throne. I look down the above mentioned receptacle with light and can can see if it needs attention. Flashlight needs to be large enough to not be able to get in tank.


A Fairly discusting article, but one that needed to be written!

Our sensors haven’t worked in our RV, pretty much since we purchased it. And WE have tried everything! So, we generally go by Sound! Listening for that (here it comes) “infamous PLOOCH”, is usually a good indicator. We only have a 40 gallon tank and can safely go about 4 days, since both my Wife and I use allot of water. “WATER IS YOUR FRIEND” when it comes to Waste Management!

Have a Great Monday!