I found a propane water heater in " The sportsman's guide" catalog.

it puts out 1.4 gallons an hour and uses about one pound of gas per hour.

it is 12x15x4 inches and includes automatic, battery powered lighting.

the down side is that it must be mounted outside.

The plus is that it's only $119.

Maybe some clever person could figure out how to mount it on the tongue or bumper.

Their phone number is, 1-800-888-3006 for ordering, 1-800-888-5222 customer service.

I've seen these water heaters available elsewhere, too.

They're a tankless on-demand kind of system. I suppose they could be adapted relatively easily to the RV for nice weather use. I'm not sure how well they'd work in colder weather though, being mounted outside.

I've seen those too, and wondered whether I could engineer a shell insert for them that could turn one into a good trailer water heater. Standard 6-gallon propane heaters have a number of disadvantages, not the least of which is they take up a lot of cabinet space and hold 50-plus pounds of water (6 gallons) that can't really be used to drink or bathe in because there's no easy way to get the water out of the heater without pumping more water in.

So a converted instant water heater could save space and weight. And because it has an electronic ignition you'd never have to worry about going outside to light it on a cold, dark, rainy night.