That's a nice project, Travis. It has been done a number of times, with slightly different details. Most folks put the switch on the sink apron, near the switch that turns on the water heater's propane heater. Some folks add a pilot light as a reminder that the heater is ON.
And I went whole hog by adding a relay, so that when you unplug the camper from shore power, the relay drops out. Now if you drain the water heater, and absently plug in shore power again (at the next campground, for example), the water heater won't power on again, saving you from a burned-out element. To turn it on requires pushing a button, mounted on the sink apron. I called this the "Super-Duper Automatic Water Heater Turner-Offer". Details are here on the forum at
https://www.trailmanorowners.com/for...php?albumid=74
Bill