Is it the black tank that shows 1/3 full all of the time or is the gray tank also included. I had this problem on my black tank and all it needed was a good cleaning.
When we went on a camping trip, I filled the black tank 1/3 full, then dumped a bag of ice from the local convenience store into it. As we traveled, the ice and water sloshing around in the tank cleaned the sensors.
When we got to the campground, I emptied the black tank before setting up and the sensor was reading empty. Have not had a problem since.