This is probably not the answer you are looking for, but do you really need and want a hot water tank?
Twenty years ago, we bought a 26ft Avion that had spent several winters parked under the trees without being winterized as the original owner had developed a serious health condition. The price reflected this fact and I was not surprised to find tank seams split and copper tubing split from freezing. We did retain the cold water system, but deleted the hot.
After many interesting and fun trips while our kids were still home and now that we are retired, we have used our Scamp
to explore the southwest. We are planning to re-fit the Avion for just the two of us to winter at some of the places that have interested us. However, a hot water tank is not in the planning. We never missed it.