we've ran into this problem with our Anderson 4-way 'kantleak' valve, so I just installed an in-line cut-off valve between the suspect valve and the water tank. I keep it closed when I don't want the 'leak' to fill the tank, and open it when I do, or want to move to 'fill water tank' setting.
I find that allowing the 'leak' to slowly fill the tank while you can use all your facilities inside is much more pleasurable anyway. I then cut it off when it's near full.
the cut-off valve is the same type as behind your toilet at home... and easy handle to turn.
'14 Palazzo 33.3 bunkhouse 34' diesel
KingTailgater2 Dish HD, 100w/5a SOLAR, BlueOx
100,000+mi since '14 - US, Alaska, Canada - Hiawassee, GA! (also, '14 Gulfstream Amerilite and '07 ForestRiver Rockwood) : )