Also, Tank sensors are notorious for being off. They get cluttered with debris, and rarely read correctly. Turn your water off. Hold down on your toilet flush pedal, and shine a flashlight down there. If you see the bottom of the tank, shes empty. If you see liquid, and its not draining when you pull the valve lever, I would take a hard look at your knife valve. Make sure its opening and closing properly.
As stated above, your tank valves should be closed until tanks are near full, and then opened to empty. Leaving it opened while hooked up just allows solids to accumulate in the tank.... If your gray tank is full, water will start coming up the floor drain in the shower before anything else.
2010 sunset trail ST26RK, 2012 Nissan Titan SV crew cab, equalizer hitch