FYI.
I read all the manuals I got with my RV, even read every single part of stuff I already knew about or so I thought.
What I found out about sensors 'directly from the manual' for the tanks:
Certain minerals found in water all by itself can foul your sensors and keeping them from working. And since we all use CG well water often these sensors can stop working from just the minerals in the water.
Which is why I use Thetford's Sensor Cleaner at least once a month.
Black Tank: I use Campa Chem and whenever I dump I fill up with water and dump again and then put several gallons in to cover the bottom.
Gray Tank: One quart of lemon cleaner every other dump.
My sensors work ALL the time!
So don't just lay down and 'accept' that sensors don't work because they DO....with a little bit of sensible maintenance of the tanks. :C