Holy cow!!!! 122 gallons of fresh water? You are packing some serious hydration. I am not sure exactly how much I have. I think it's a 40 gallon tank. With three of us in the RV, we can make that last the weekend but not much more.
The grey fills up quick and we don't have a transfer pump. It's on the list of items I might add.
As to gas usage, I would need to see published comparisons. When I looked into home based tankless systems, the price difference on fuel usage was non-existent. I can't imagine why there might be any difference in results when looking at RV based systems.
I run a tankless commercial (180 degrees possible) heater for my hot tub. Everyone said "don't do it because you have a salt chlorinator", but it has been working AWESOME for a couple years now. I flush once a year with CLR and it keeps the exchanger flowing nicely. For the right application, tankless is great.
Jose