Interesting idea to use the inverter a lot more to save on propane when you have enough solar. Do some of that now in a small way, eg can use either the electric kettle or the stove top to boil some water.
It does depend on how long you are off grid to matter any. Also need a water heater that has electric heating. (Ours is propane only)
Do those 12v heaters you can stick in the water heater work with an Atwood with its plastic drain plug, or only with a Suburban with its metal anode plug? The plastic plug in ours is sort of behind the gas fittings, not a straight-in to get at it either.
Not too sure where they stick in:
https://www.solar-electric.com/diversion-load-water-heating-element-24-48-volts.html