We have pretty much given up on those internal sensors. We just learned to tune in on the sound that the water makes when flushed down. There is a difference as the tank fills. We have also learned about when the tank should be getting close to full so we tune in a bit sharper for the change in sound. Or one can just peer down the hole with a flashlight, don't drop it, though.
I had to install a new black tank in our current Safari Trek. I installed new internal sensors. Unless they are flushed/rinsed off with a jet of water they will not show empty after draining.