Don't know how yours is wired, directly into or uses a plug, but if you can tap into electric line going to the water heater (before that rocker arm switch), you can run the wire anywhere you want in the camper. Then go to your local RV parts store or dealership and have them get you a new interior switch used for water heaters. Install it anywhere you want inside the camper. The problem will be finding a way to path the wire so it's hidden and going through walls may be an interesting challenge.
I added switches to the main house lights in my new 5er. This was the first modification I did. The house lights switch are located behind a door panel with all the other switches, and very high up. I tapped into the wiring and ran 2 new wires and located a new switch (2 of them) in a lower, more convienent location. However, in order to do so, I had to remove the microwave and remove part of the interior of a couple cabinets, and drill holes inside the cabinets to locate the new switches. End result is, I can use either switch, but the original has to be turned on all the time for the new second switch to work on-off. (Not in a series). It works. I got the new switches from my local RV dealership that sold Keystone RV's.
I also re-ran the wire and a switch for the bathroom lights. I had to get a second switch, and just left the original one in the bathroom there, dead, (removing would have left a hole), and the new one is now in a good location, and works just fine.
So you, you can double switch your water heater. I advise getting the right one from your RV parts store and take the time to route the wire wherever you want to put it. FYI... routing wires in RV ... you REALLY have to get creative. Wall are not like they are in houses!