I went with
SeeLevel Holding Tank Monitoring System for 3 Tanks Model 709. The sensors are electronic and stick to the side of the tank. I installed this in the last 2 units we've owned and find them very reliable and accurate. On our current unit I even have a control panel on the outside near the dump valve door. No more trekking in n' out of the unit at the dump station. If there is a flush hose present I can add my detergent and softener down the john and add a precise amount of water to the black tank with the hose, eliminating hauling in a bucket.
