You shouldn't be talking about pressure on the tank side. If yours is set up like the one I dealt with in my previous fiver, the valve was actually high up behind an interior wall, so that when you turn off the fresh water pressure the water on either side of the check valve will naturally drain to the tank on one side and the incoming water would drain back down the pressure side. There is no reason there should be back pressure from the black water tank to the check valve.
Mine had a sign that said to always open the dump valve while flushing so I can say there should be no way to have back pressure. Having a check valve at all is being ultra conservative. The water on both sides of the check should be clean at the point when the hose is turned off or disconnected.
