OB, beat me to it, nice job of showing the often hidden electric switch. Electric is a little slower than gas but free when you have hook ups. The electric will also work when the generator is running. If the gas is running you will not only hear it but be able to feel the exhaust heat rising up and out of the water heater compartment.
IMHO: it is a good idea to be sure the gas water heater function is turned off before entering a gas station as there is a large flame the goes off with ignition.
I am curious how some you heat your water electrically while running down the road. Your batteries won't/don't do it so are you running the generator?
BTW: There is no such thing as a "Hot water heater". if the water is hot you don't need to heat it. :B :S :B
