I looked at the eletric tankless hot water heaters at length, but I could not find any that draw less than 30 Amps. Many AS have a 30 Amp hookup, so you prettymuch could only be running your hot water heater, and nothing elese. No AC, power inverter ETC.
You could go with a small tank eletric hot water heater, but you will not be saving much money. You could hook this up, and only use the elec side, it's pretty much about a cheap as you can get. If you get one w/o the direct spark it is bit cheaper.
SUBURBAN WATER HEATER - SW6DE - 6GAL GAS/ELEC W/DIRECT SPARK
...Regularly $548.00 On Sale for Only $307.95 From PPL
Also, do you have one in there now? If it does not work, and it is eletric/gas, changing the heating element is fairly simple.
Tankless is the way to go if you don't want to run out of hot water, if you have a trailer full of kids, or just really like that long hot shower. If your at a camp with full hook ups, then a really long shower is possible. If you are filling your grey tank, it will be a shorter shower..