We have a 2007 Winnebago; it uses the True Level tank monitor system. The outside pads that attach to the tank are Mirus detector cells. Mirus detector cells use a micro electrical field to detect liquid level.
The installation process says to keep all metal wiring or harnesses 1/4 inch to 1/2 inch away from the Mirus pads or it causes interference. Perhaps the bouncing around caused a wire to move near one of the cells. Its probably a long shot but worth mentioning.
We purchased our MH used and the tanks were a bit flaky at first; reading 1/3 or 2/3 full when empty. We do not empty at the campground, but drive to the next campground so the stuff in the tank has time to slosh around and breakdown. After a few trips our tanks now read correctly.
With the Mirus detector cells I don't believe we have an adjustment on the Red circuit board. I made the following measurements on our unit.
With the Systems monitor panel removed the test results at large white connector was as follows:
Ground - Pin 3 (white wire): Neg battery terminal connection
Fresh Water - Pin 4 (red wire): Empty = 4.94 volts, 1/3 = 4.81 volts, 2/3 = 4.69 volts
Black Water - Pin 5 (brown wire): Empty = 4.94 volts
Grey Water - Pin 6 (grey wire): Empty = 4.94 volts, 1/3 = 4.80 volts
If you get these readings at Pins 4, 5, and 6 then it appears that the Mirus Detector Cells and the ground are in good condition and the red circuit board is putting out the correct voltage to the cells.
You can also get these readings if there is stuff coating the inside of the tank.
Our previous motorhome had a tank flush system but it never seemed to clean all surfaces of the tank and we could still get false readings from time to time.