Can't help you on the drain valve location, but I remember them being pretty easy to find in nearly every case.
Regarding isolating the city water from the gray tank, by gray, do you mean fresh water tank? If so, there are check valves built into the water pump for that purpose. If you're leaking into/filling your water tank when hooked up to city water pressure, they're either shot, or have a piece of crud lodged under one of them.
Although I don't remember them coming this way from the factory, there's also a possibility somebody has installed a valve that allows the fresh water tank to be filled from the city water pressure.
Sounds like you're up for some time spent on your hands and knees getting to know your coach a little better? ;^)
Best of luck - Al (I worked on these when new, but changed occupations in the early 90's, so it's been a while!)
1997 37' HR Endeavor, 275hp Cat, Freightliner
03 CR-V Blue Ox, Ready Brake