You can clean the sensors, so they work...until you start to fill it up. They are very unreliable and everyone knows it. Yes, you can buy expensive sensors, which may work a little better, but they too will become unreliable. You will just need to begin looking into the downtube (from the toilet) every day or two to get an idea where the level is. Once it gets to a level, which you are comfortable with (4-6 inches from the toilet), dump it. You will then have a pretty good idea of how often you need to dump. This timeframe is a guideline, and needs to have some common sense added to it. If someone gets ill and uses it more often, then you'll need to dump sooner, etc.
You an also use your grey tank as a gauge. It will likely need to be emptied sooner than the black tank, so when the grey tank needs dumping, dump the black at the same time.
Don't rely on the black tank sensors.
Ben and Doreen
Home Away From Home - 2017 PT Crusader 315RST
TV - 2008 F250 Lariat 4X4 6.4 Diesel
42 nights camping in 2015
49 nights so far in 2016 (Still Not Enough!)