When the water lines have been pressurized, either through city water or the water pump, open the kitchen sink hot water faucet. When the water runs evenly, turn on the water heater. Close the hot water faucet and wait about 3-5 minutes. You should see plenty of scolding hot water at that point.
2014 Thor Chateau 22E Class C
Ford E-350 SD DRW 158" Chassis w/6.8L V10