Hi,
On demand heaters are available but so far only one of them appears to work well. There are no electric ones available so instead of using the campground's electric (often free) you will have to use your own propane (not free).
Recovery time on 120 volt is 90 minutes (dead cold to cycling) and on propane 30 minutes. Using both brings that time down considerably.
I find that 15 minutes on the electric setting will yield a warm shower. If the tank is fully heated in the evening, and turned to off, then by 9 am there is still enough warm water for a shower or two.
I'm fortunate to have a ten gallon tank. I've never run out of water that was warm enough to have a shower with.
The very nicest water heaters for self propelled RV's include a way for the engine coolant to heat up the water in the water heater.