The shape of the tanks do not lend themselves to accurate readings.
They are broad and shallow. The distance between the probes is small and easily bridged with toilet paper or other crud.
There are numerous compounds you can add to the tank to improve the situation but I have found these to be short lived.
I had as good, albeit temporary, results with a bag of ice dump down the stool before driving to the next camp ground.
Like most folks you just learn how full it is, or you look down the toilet. We know when our gray water tank is full because it will back up into the bath tub. :)
