artnmel,
I have moved your thread into the Cyclone area where more Cycloners will see hopefully chime in. In the mean time, can you add a picture of this switch?
If the switch is just light a standard residential style toggle-type light switch but with a red lighted handle, I would guess this is to turn on the electric heating element in your water heater. Never turn the switch on unless you know you have water in your water heater tank. Test this by turning on the hot water tap. If water comes out, you have water in the water heater tank. Then and only then, may you turn on the electric heating element for heating the water.
I don't want to dissuade you in any way from using the electric heating element. I'd venture to guess that most people use theirs more than heating the water with propane as most time, nightly rental sites include the cost of electricity.
Do you by chance have the tank heating pads option? I have never seen this done but there is s slight chance this is the switch. It is usually found with all the other switches though.
Jim