I know I'm coming in a little late on this thread, but hope to buy a bus soon and need to think about these things.
I like the idea of a tankless water heater, but was unaware of the pre-water temperature requirements. I don't want to go with a traditional water heater because of the constant energy consumption (be it gas or electricity) created through maintaining temperature.
BUT... why not combine the idea of a traditional water heater with tankless heater? Sounds silly, I know, but bear with me... Traditional heaters maintain water temperature through a combination of heating element and insulation. But a tankless system heats water "on demand" as it's flowing. Why not take an insulated tank (like a large xtreme cooler) and use it to store the water that will be pumped (or gravity fed) into the tankless unit when needed? The insulation will maintain "room temperature" that allows the tankless to produce the desired "hotness". A two tank system would bring cold water in from the source to a non-insulated tank that would then feed the super-insulated tank via thermal convection as the "warm" water achieves room temperature.
Alternately, if you live in sunny climates, I would suggest running lots of black-painted PVC pipes along the length of the roof to collect heat from the sun during the day. If you're in a colder climate, use greenhouse poly to cover the pipes and help trap the sun's heat. With this type of system, shower in the evening and you'll have the opposite problem of trying to keep the water COOL enough!
Just my crazy newbie ideas.
Any chance they might work?