We have an instantaneous heater, and wouldn't be without it! It makes you human!! The small units do not get very hot, so can be hung just about anywhere with no worry of starting a fire. The only part that gets hot to the touch (momentarily) is the top vent. Our trailer has brackets built into it that work perfectly to hang the heater. It connects to a 5-gallon propane tank, with RV water hoses to connect from an outlet on the trailer to the heater, then from the heater to a hand-held shower head with an on/off valve. There is a 30-gallon tank in the trailer, and a 2.5 gal/minute on-demand pump (and of course a battery). You can get two comfortable showers out of 5 gallons (meaning not just a quick rinse, but one you can actually enjoy). We normally set up a TexSport shower tent, it's 4' square so plenty of elbow room. The water gets quite hot! Also good for heating dishwashing water.
Note that there is just one hose to the shower; you adjust the temp using controls on the unit that limit flame size and water flow rate. You could build one of these into a compartment in the side of a trailer, and just open the cover door when it's in use. But it's very little trouble to do it the way I do.
