You might try something like this (or some variation) I had to repair my water tank winterization valves on my MH.
Water heater bypass valves | 2001 Coachmen Mirada 300QB
I always carry a push on PEX valve in my spare parts box. I can always cut a supply tube and insert this valve into the line to shut off an offending leak and still be able to operate the rest of the water system.
WATER LEAK on the water heater, look at the supply tubes in and out. The drain plug is also a common source of leaks (do not replace with steel plug, use nylon or brass). And the over pressure relief valve can leak, you should exercise the relief valve a few times a year to ensure its not frozen shut (pull on the lever and it will squirt water at you_)