We carry a fairly big thermos and find that if we heat water on the stove and fill the thermos before turning in. The water stays hot enough that we can mix it with water from the tank, and have instant warm water to wash up in the morning.
Interesting technique. I so enjoy snuggling back under the covers for another 10 we've adopted a plan that heats the cabin at the same time. I sit our Coleman Propane heater on my (sliding door) side on top of the port-a-john (behind the passengers seat), while the Admiral has a big pot of water sitting on the stove put there the night before. I can reach out and hit the heater starter, while she lights the stove. By the time the water is hot enough for bathing, the cabin is warmed up from both heater & stove going the same time. In the meantime, we enjoy an extra 10 minutes under the blankets.
I should have mentioned that our method is more appropriate to the plainer style of Westy - icebox & sink only. It lets us have warm wash water in the morning without having to get out of the van. Your idea is great for the better-equipped version and accomplishes the same thing. Must be nice.
We do the same thing we do inour remote cabin. We make coffe in a thermos carafe. We then put the carafe in a "cozy" made from insulate foam. The carafe won't keep it hot enough over night but the foam cover does the rest. (At our cabin we put it on top of the stove over the propane stove pilot lite.) Same deal for hot water. I care much more for coffee first thing though,,
I like the hot water in the thermos idea. take it one step further, and use one of those air pots. we use them in our coffee shop, they keep hot liquid hot overnight, then just pump the top and out it comes.