The sensors in the black and grey tanks get crudded up. That is common to all RV's that use the type of sensor that goes inside the tank. There is virtually nothing you can do that will keep them clear and reading like they were the very first time the tanks were used. Most of us know what our use time is and just dump the tank when we hear a difference in the sound of the flush. Except in dry camping or while building up grey water to flush the sewer hose we leave the grey valve open.
There is a solution to your problem but it will mean installing a new set of sensors that work from the outside of the tank. There is a thread here on the forum that talks about installing them and gives the brand name as well. (
http://www.5thwheelforums.com/forums/viewtopic.php?t=6 ) It is called the seelevel II instalation on this same area. I don't have them on my rig but have been giving them very serious thought as the only sensor that is "accurate" is the fresh water tank on our 6 month old rig.