If you have an Atwood propane
only water heater, you can buy and install a Hott Rod electric water heater element to be able to use AC power when available, without using the propane
It voids the warranty for the Atwood, but mine is 15 years old so no problem.
The heater element kit was about $40 from that auction site and took about an hour and a half to install - mainly because it kept leaking (dripping) at the screw-in.
Contrary to what the instructions say, I ended up using teflon tape over plumbers putty (a crayon type thingy that you scrape over the threads) and tightened more than they said it would take to get it sealed.
You then run the wire through the silicon sealant alongside the propane
pipe into the trailer.
You use a knife and cut a square in the cardboard tank cover, open up the fiberglass insulation, and stick the thermostat onto the tank. Then fold back all the fiberglass and cardboard and use duct tape to seal it all up.
I already had an AC outlet near the tank, so, after refilling the tank, was able to plug it in.
It's only a 400 Watt element so will take about 3 1/4 hours to heat the water from 40 to 125F if in the 6 gallon size, I calculate.
Use more silicon over the wire hole to keep fumes from entering the trailer when on propane, and you're good to go.
Just another gadget to keep improving my big little 'egg'!