As stated above, the sensors on the holding tanks very seldom work right. Maybe the first day after being shipped from the factory, but after that, it's anyone's guess.
Reason? Stuff sticks to the sensors giving a false reading.
The solution is really simple however. All you need to do is dump your tanks completely. Rinse them out by adding a full tank of water via the toilet and dump. This cleans out any excess stuff that did not get dumped out initially.
The next step can vary, and maybe do all of these procedures.
The simplest is, now add about 5 gallons of water to the black tank, a bit of dish washing Liquid to the tank. If you want it to smell better, add a cup of bleach, and take the camper for a 20 minute drive over bumpy and curvy roads with lots of stops and turns. This will slosh the contents of tank around, sloshing and cleaning the stuff off the sensors.
When you get home, or your next camp site, drain the tank. You don't want the bleach sitting in there long term. If yo don't use bleach, dish washing liquid won't hurt anything.
Now, if you can't drive, or even if you do, there is an alternative. Assuming your black tank does not have a black tank flusher attached to the trailer, get a toilet wand from any local RV shop or even WalMart. Attach it to a garden hose, shove it down the toilet, turn the water on, and spray the dickens out of the tank, letting it drain as you do. The high impact of the water hitting those sensors, and the inside of the tank, will pulverize anything left clinging to the sensors.
The sensors will work again correct (but only for a while). So the secret to keeping them working is to empty all the contents of your tank before hitting the road every time. Then add about 5 gallons to the tank with a bit of dish washing liquid and drive to your next destination. If you do this (without the bleach), not only will your tanks stay clean, you'll never have smells, and your sensors (should) work.
If your camper is stationary, then use the toilet wand on the garden hose after ever few dumps, or you notice the sensors not working correct again.
This is the least evasive, an in my opinion, the best way to keep the sensors working AND the tank clean.
If this does not work, then there is something wrong with the sensors. If that is the case, then consider switching or replacing the sensor system as suggested by the above posters. No need to spend money yet. Try it, see what happens.
