Sensors are notorious for malfunctioning. First thing first. You can leave the valve open on the grey water (aux) tank when you have a sewer hook up. Do not however leave that valve open on the black water (toilet/main) holding tank. You need to dump that tank with as much liquid as possible. If you don't, the solids and paper will build up in the tank and you will have some serious problems.
Going back to the aux tank. Typically if the tank fills, the water will back up at the lowest drain in the trailer, which is the bath or shower drain. My advice is to close off the aux valve and run the water in a sink and start to fill the tank. Two things will probably happen. First you will find that the issue is a bad sensor and secondly when the tank finally fills (watch that shower/bath drain) then go out and open that valve. You will hear that water rushing out and you just might have cleared debris off the sensor allowing it to register correctly. Typically the black water tank sensor goes bad first (due to paper and other solid wastes). Typically if the grey water valve is closed, you will find that you tend to dump a lot more liquid into that grey tank than the black.
'04 Classic 30' S.O.,'03 GMC Savana 2500,'14 Honda CTX 700