What everyone else said... Dirty sensors are a common problem.
I'd add that by adding a clear, short extension in your sewer system you can see what is being flushed out of your black tank. You may need to flush longer than you are. Even so, you may not (if ever) get the sensors clean enough to work as well as you'd like.
If you have a straight view down into the tank, shutting off the water and holding the toilet dump valve open will let you see down into the tank... shut off all exhaust fans first!
If you haven't been, you also need to add water to the black tank after you dump to be sure 'stuff' is not building a 'tower of terror' on the bottom of your tank.
This summer we stayed in one place for 3 months as campground hosts. That was a first for us. We had to do two things differently because of staying on one spot:
1). Run the black tank flush longer (several times for ~30 seconds or so) than we did when we were traveling frequently.
2). For #1s, DW put her toilet paper in a trash bag. I don't know if this is necessary, but she was willing and it seemed to help.
Even with the above, our sensors don't read like new, but I know we are not building up residue in the tank that will be difficult to remove.