You can try all kinds of new-fangled ways to try to clean the sensors, but the easiest and simplest is to simply drain the tank completely. Then add about 5 - 10 gallons of water. Now dump a couple cups of dish washing power down the toilet and go for a 30 minute drive! Once you get the camper back home, drain the tank again. Not only will it clean the inside of your tank, but it will also clean the sensors. If this does not work, then your sensors are probably shot!
If you want to use Dawn Dishwashing liquid down the toilet instead of laundry soap, Dawn works VERY well also. Pour a 1/4 bottle of Dawn into the toilet with about 5 - 10 gallons of water and go for the drive! Clean as a whistle! And very safe and sewer / septic tank friendly too!