Most of the holding tank drains I've seen would make it impossible for the gray water to back up into the black water holding tank regardless of the valve settings. The black tank on our coach drains straight down through the 3" pipe while the gray tank is teed into the 3" drain. It is physically impossible for the gray water to back up into the black water tank. That would require a 3rd valve be added to the 3" drain below the tee fitting, and even then I doubt more than a few gallons would actually get into the black tank. A tank flush is much more effective.