meh...we'll sometimes take a baby pool for the GF and set it up on the beach. Sun gets it pretty warm or we can just add some hot water off the camp fire that's always going.
I never thought about it but my buddy has a coleman heated shower system. We could just use that and put both the spout and the source tube in the water to recirc the hot water. Really the pump, battery, hoses and element isn't THAT big for it.
hmmm...guess I have another hillbilly house project!