Tankless Water Heater

I see a 2-gallon-per-minute point-of-use tankless on the Home Depot web site.  But it needs 240 volts and 50 amps.  You might be able to run it from a "big rig" outlet at a campground - they supply 240 @ 50 amps.  That's pushing it.  You wouldn't be able to run anything else at the same time.  Even the converter would have to be off so you would have to run the lights, exhaust fan and furnace from the battery while you shower or wash dishes.  No A/C in the summer.  No microwave.  Finally you would have to change the cord and plug to the power pole, and add a separate circuit breaker panel.

I did find a 120 volt model elsewhere that needs 27 amps so you might be able to run it from the existing electrical system - and use the converter at the same time as long as it isn't charging the battery.  Nothing else though.  And its water flow has limits - it can do 1/2 gallon per minute if you need to raise the water 41 F degrees, and with a starting temp of 50 F that's only 91 F on the output.  That might be enough water flow for the low-flow showerhead in the 'pod, but usually people like 110 F water for a shower so 91 would be cool.  Start with 30 or 40 degree water from the faucet (likely in the winter) and your shower would be cold at 70 or 80 degrees.  If you raise the water flow to 1 gallon-per-minute it will only raise the water temp by 20 degrees so that is entirely useless.

I would stick with the existing water heater.  If you use the propane and 120 volt systems at the same time you get a very good recovery rate with no worries about flow rates.  Even if you only use the 120 volt system you would do better than the tankless systems above.