C WHITT is correct, either or both at the same time depending on your needs for 'quick' heat up, and lower cost 'constant' temperature maintenance.... or neither if you like 'navy' showers.
This, of course, only provides electric heating when you are plugged in OR on generator...though you can leave the 120v switch on because it won't matter until you plug in or run the generator anyway. The 'propane' switch, though, should only stay on as long as you want the propane to fire up and heat the water when needed.
We use the propane sparingly, but mostly when wanting a shower quickly after arriving for the day, or when a few showers in a row are happening. : )
two Campers, two Electric cars
former diesel pusher traveler