I'm trying to make one as well for my camp. I need hot water for dishes and clean up too.
I think the problem with the propane heater is it will get too hot. I think you need to get the water circulating when it reaches a certain temperature.
I was looking into using copper tubing against a blackground and getting the sun to warm up the pipe with water in it, and then when it hits a certain temperature it would circulate into a storage like a thermos. But the price of copper tubing blew that idea out the window.
back to the drawing board.