[Festus2]

trackstand -
Readings from the black tank have always been known for being inaccurate. They often start out giving true readings but soon develop a "mind of their own". Most folks spend a considerable amount of time and effort in keeping their black tank as clean as possible and; despite their efforts, the sensor and reading doesn't reflect what is actually in the tank. It could be, for example, "squeaky clean" and have a couple of gallons of fresh water in it, but still shows 2/3 full.

At first, people find it frustrating and ask the same question as you but soon learn to live with the fact that it's just the way it is - regardless of how much money you paid for your RV.

All sorts of devices and "tricks" like travelling with ice cubes in the tank have been tried over the years to have the reading accurately reflect what is in the tank. Almost all have failed --- the tank may be clean and empty - but the reading doesn't show that.

In the end, it's not worth getting too excited and; like many of us, have learned to rely on other, less sophisticated methods of figuring out when the tank is nearing full and it's "time to dump".