The problem is not the sensors being dirty but is due to the tank walls being gunked up and fooling the sensors.
Even the non-contact sensors on the outside of the tank can be fooled if the walls are cruddy enough. SeeLevel even points this out on their web site.
The SeeLevel web site says "If sludge buildup in the tank becomes extreme the gauge will cease to operate (the tank will always read empty), so by monitoring the signal level the tank can be cleaned before the buildup gets excessive".
Anything you do to help keep the tank walls clean may help.
Flushing, adding some detergent and water before you drive for a day or so might help.
I have the TrueLevel non-contact system that Winnebago went standard with in 2005 and right now my black tank reads full when empty. The gray tank shows 2/3 full when empty.
Over the eleven years we full timed I tried everything people say will work and nothing worked.
If I pressure wash the inside of the black tank (have done that twice), the level will be measured normally for a few months and then start to lie.
I think the problem is worse for full timers and that may be why some weekend or short term campers find that detergent or some other method works for them.
I just dumped it every ten days and didn't worry about it anymore.