If you have the Ventline sensors that go through the side of the tank and they are installed at the correct height they may have potentiometers on the display board below the LEDs that can be adjusted.
As side note the problems with the sensors are not due to crud on them, but rather crud on the tank walls that is wet and fools the sensors. This happens even with the external sensors like Sealevel and TrueLevel (installed by Winnebago as standard since 2005).
It seems to occur more with rigs that get a lot of use, like full timers.