Yes, camper water systems are set up to feed everything from the city connection -- some people never fill or use the fresh water tank at all.
The water heater is plumbed downstream from the point where the pump and city supply lines tee into. This means the heater will draw from whichever side has pressure.
However, there are valves in the system used to isolate the hot water tank so it can be drained for winter, etc. These are most likely oriented correctly so that you can switch back & forth between tank & city without worry.
You should however familiarize yourself with the plumbing layout. There should be schematics in your manual, but it is also worthwhile to take a good look at the plumbing and follow all the flow options with each valve setting and water source.
