I have been refurbing my 62 Airflyte and wanted a HW tank even though it didn't come with one originally. I had to decide where to put the dang thing and didn't want to give up all my storage under the sink. I decided on an Atwood 6 gallon gas unit that measures 12x16x16 or so. My street side rear storage hatch was pretty beat up so I had the idea of cutting it down to a smaller size and putting the heater in the rear of the storage area. It fit great under my new bunk and the small hatch is where we are mounting a shower valve and hand held shower head for some quick camp showers. The spaces worked out great. Of course I had to add a new water tank and pressurized system but that is great. I am a contractor and have the PEX tools for plastic piping and that is making for a tidy and strong plumbing system that is easy to snake through touch spots.
I will take a pic or two when the system is complete.
Universal, the cool thing about the tankless is that there really isn't any water that sticks around inside the unit. All we do is to lower the shower nozzle below the level of the unit. In the wintertime, we just leave it on the floor and make sure the water tank is empty and we've drained any remaining water from a city source. So far, no problems.