I keep my tank sensors functioning by:
1. Using the black tank flush every time I dump the tank.
2. Always adding a black tank additive which also eliminates odors.
3. Adding fabric softner will aid keeping the sensors clean.
4. Check the tightness of the wires attached to the sensors .. if they are loose or corroded, the sensors won't give a true reading.
5. NEVER use generic toilet paper as it does NOT dissolve well.
Also the use of RV/marine toilet paper is NOT necessary as Charmin and most other 'home brands' will dissolve as well as the more expensive RV/marine toilet paper the is thin and doesn't really 'do the job.'
Simply watch this test:
And if you find your sensors not giving a good reading, simply fill your black water tank full when home and leave it sit for several days - you can add some dish soap if so desired.