....we're seriously looking into a tankless "Truma" water heater / furnace (a very compact dual unit) to mount under our kitchen counter, in our 8-foot Outfitter pop-up. I saw one demonstrated at the Overland Expo East (Asheville, NC) in October. It was Rory Willett (manufacturing a new camper line-up, is the North American installer for Truma) who had his new truck camper design with a Truma installed and working (only 1 small ~3 inch hole through camper wall needed). I was extremely impressed. Going to a 90%+ efficiency Truma from the old technology water heater / or furnaces, would be comparable to going from an internal combustion engine to a hydron fuel cell engine (metaphorically).
Rory related a price for unit and install in the $1500 range...