[DavidBall]

depending on how you operate your coach (Park or boondocking) would be to choose an alternative. If you drive or run your generator quite a bit you could opt for a hot water tank which has an internal (marine) coil wherein you could circulate the hot water from your engine or generator and make hot water while that source is running.

Another choice would be, a solar heat exchanger on the roof, or remote (which would require a small circulating pump) which would ask Old Sol(The Sun) to heat your water for you. The downside of that is a well insulated storage tank, of some capacity, to retain the heated water.

There doesn't seem to be any easy or inexpensive choices that I know of.