Betty, I've not seen them. If they ran off the battery, they'd be VERY hard on it. If propane, then they'd work very well I suspect and if 110V, they'd be OK, unless you like to boondock like I do.
I have seen both propane and 110v advertised just wanting feedback before purchasing! Our hot water tank can be cantankerous sometimes, as well as my honey when it won't light!! Definitely a want not a need
Just went to a site to read up on them. You can definitely get them for propane and different ignition systems. Drawback is need for pressure for them to work. In other words they get hottest when you need full pressure. The less you open the tap the less temperature rise. That's for a dwelling. They do make small portable ones which might be worth the effort to check out.
When I owned a Coleman Utah pop-up, my gas water heater was on the fritz, I was in a hurry (of course) , my plumber friend gave me a used Point Of Use or On Demand heater. 120 volt. I wired it in and long story short, I never took it out, worked perfect when hook to shore power, no tripped breakers etc. Told new owner the story, he liked the idea. Just had to turn on the breaker during set up, off when leaving.
Treeclimber