What some folks don't know is that there are many different configurations and dimensions of black tanks out there. Some drain super easily and some are super hard. To say that some are being way too anal isn't fair.
We have one of the super hard ones and it is mandatory to get the tank spotless inside when breaking camp otherwise there is a problem with the tank building up a pyramid of poop too soon and we don't get enough use out of the tank. The first thing I do when breaking camp is pull the black tank handle. I use our tank rinser for around 10-15 minutes and if not enough, will fill the tank, dump and do another round with the flusher. Besides seeing what's coming out the clear elbow, I know the tank is clean when I can look down the toilet and see that the bottom is spotless along with droplets of yellow ABS glue. Even then, there still could be slime at the curb and street sides of the tank that the flushing process can't clean off. Getting the tank well cleaned helps reduce false readings but not necessarily eliminate them.
Wouldn't want to have to do this process at a dump station tho.
On our first TT, all we did was a dump and a quick flush with the Valterra backflush elbow and that was all that was needed. Never ever had an issue with that tank. Ever. Now it's the opposite with our KZ TT.