Our used motorhome came to us with an electric\gas water heater..Problem is the electric part is a HOT ROD add on,with an extension cord that has to be plugged in around front of the kitchen cabinet....
If you're going to run romex for a new water heater, why not just run romex to a switch in an interior cabinet near the water heater and then romex to the heater. Route it into the heater near the edge of the water heater box.