[Chuck v]

In your control panel where the tank level indicators are there should be both a water pump switch and a water heater switch. Next to the water heater switch is the indicator for the pilot light status. This water heater switch is the one that controls the gas heating function, and turning it off disables the gas heating burner in your water heater (most likely a Surburban but possibly an Atwood appliance...) With the gas heating switch off, the electric switch under your sink should allow shore power to heat your water electrically. There is another small rocker switch on the water heater itself that needs also to be in the ON position to allow electric heating of the water -- be sure that is also in the correct position.