I had a similar problem with myGalleyGrey water valve. I posted the "fix" on the forum undeer Leaking Grey water valve. To summarize - it takes a lot of patience, a few simple tools. I also installed a third valve near the end of the sewer system. This provides a positive stop against any of the other valves leaking by - plus - it is eay to get at. I bought the extra valve from an RV dealership - you will need: the valve kit, hack-saw, glue, sandpaper - and a screwdriver - in my case, I believe it was #2 Robertson. As for getting at the leaking valve, please see the earlier post (I believe it was late July 09).
Roselyn and Darrell
2007.5 Silverado LT/EFI Live Tuned Duramax/6Spd Allison/Banks Ram Air with Super Scoop/Edge CTS Gauges/Air Lift Load Lifter 5000 Ultimate Air Springs
2014 335ss Cruiser Provincial