RV water heaters are so small that I don't see the advantage of a tankless and wonder why the OP wants one.
It only takes 10 minutes for my tank to heat 8 gallons up to very hot for a shower with outside temps in the 40's. I have an electric element too and have it pre-warmed. In ambient temps around 70°F, it only takes 5 minutes. I only run my propane just before a shower and leave it off all the rest of the time, the electric element heats it enough for the dishes or the occasional need for hot water to wash face or hands.