The sensors on the tank are probably covered with deposits from the dish water. There are a number of products on the market that tell you they will solve the problem.
We use Dawn Dishwashing liquid, Calgon Liquid Water Softener and an occassional dose of Happy Camper Holding Tank treatment. The last is an organic treatment that eats though the Black Water waste and completely controls odor. See www.HappyCampers.World.com
The water softener makes the water slippery so that once the sensors are cleaned off they are less likely to hold the crud.