Doss, you have it 100% correct. Electric when the electricity is paid for with the camp site. Propane any other time. FYI, In my opinion, Propane heats faster.
And, there is nothing wrong using both at the same time.
I have found running gas on my water heater to be a little nicer and here is why. If your camper is a 50 amp system, you probably will not have this problem, but if it is 30, you need to think about this.
Every electric item in your camper running on AC power is using part of the 30 amp available power to your system. The water heater by itself is no problem. But add the air conditioner, the microwave, the television, the power supply (converter) all the electronics in your camper, and turn them all on at the same time, with a hair dryer, a toaster, a coffee pot, and an electric skillet or griddle, and that 30 amps goes pretty fast, causing the breaker to pop, either in the camper or the power pole.
Popping a breaker does not hurt anything. It's designed to do that. But it's annoying. It's solved by turning something off.
For some reason, our main 30 amp breaker in the camper kept popping at unusual times, and times that didn't make sense. Running the air conditioner and a coffee pot should not be an over load of 30 amps. But it took a while to realize, when nearing the max 30 amps .... THEN the electric water heater turned on ... it would blow the breaker.
So, we find this annoyance avoided by simply running the water heater on propane. Ever since we started doing this, we've never blown the breaker again. Nothing is wrong with the water heater, it just pulls a good amount of AMP and when your using near the Max for everything else ... well ... propane is a better choice. It just makes life a bit easier.
Your choice. Run both at the same time, you'll have almost endless hot water! Both at the same time is amazing! (Turn the coffee pot off first thought!)