Can you post the water heater brand name and model number .
Maybe we can get a full installation manual for you.
Some coaches controlled the LP and electric side with switches in different locations and over ride the microwave for power when the W/H is using 110v. Any 110v wall switches around the coach ; close to the sink maybe ; that you don't know what they're for?
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.