There's been a lot of information about inaccurate black tank sensors and I have the firefly wall display. I have learned my lessons with the black tank errors. How accurate are the propane readings? If you were in a bind could you trust it?
Mine was not super accurate but it was not off by 50% either. You should have a mechanical gauge on your tank. I look at that now and then to verify what the panel says.
As far as the tank sensors go..... a lot of it has to do with cleanliness in the tank. Even the gray tank sensors can give false readings because of grease etc. sticking to the sensors.
I use the San-T-Flush port to fill and drain my black tank twice with water after dumping and then we add Calgon Water Softener and treatment to keep the tank slippery and clean. We also treat the grey tank regularly. This keeps our sensors reasonably accurate.
You also have to remember they will report full before they are overflowing just to make sure they get dumped before they do overflow and create a real problem.