What is a hot water heater? if the water is already hot, why do you need to heat it?
I may use our 6 gallon water heater as a energy storage dump for our solar panels when the battery bank is full. It is fairly well insulated and stays hot for hours after energy input is removed. The Midnite Classic 150 charge controller has a waste not feature that is meant for this type of thing.
I think it is rather nice to always have hot water on tap even though we don't have a indoor shower. We do have an outside shower which should work fine for those really remote boondocking locations. I am 6'7" and could not design a truck camper shower of reasonable size anyway.