Since I usually fill my coach alone, I just watch the manual fill location behind the small upper door. When it starts running out there, the tank is full.
Some tanks do have an overflow pipe(NOT your tank drain) that will allow a certain amount of water to run out AFTER you stop filling the water tank. Since I have the upper manual fill, which also acts as a vent for the tank, I just plugged my over flow valve. This has caused no problems after more than four years of travel with the overflow plugged. I would only do this if you have the manual fill location above the tank, behind the small exterior door.