grey tank

About the only thing you can do with a grey tank sensor is to dump the tank completely. Then add a few gallons of clear water, a bit of dishwashing liquid and maybe some bleach and take the camper for a drive. The goal is to let the water slosh and pound on the sensors (and sides, top, and bottom of the tank), and wash free, or break free any scum build up on the sensors. If this does not work, then your next avenue will involve taking something apart to clean or replace or modify existing devices.