Not sure if this is going to work. Is this your control panel?
Full Size ImageHopefully I'm not violating any rules by linking to these images. I don't want to violate any copyrights, but I believe linking to the image in it's original location is OK. If not, moderators please advise.
If that's your panel and you do have an electric element in your water heater, here is an example of an outside switch:
Full Size ImageThere is another possibility that some Fleetwood products used in the past. My 2004 Fleetwood Bounder had what looked like a residential light switch that toggled high current loads. I believe it allowed you to select the electric water heater or the bedroom air conditioner but not both. I don't recall exactly where it was located but I think it was in the bathroom, possibly inside the vanity cabinet. This was used because the Bounder had 30A service with 2 air conditioners and they wanted to make sure only one of those items was enabled to hopefully prevent breaker trips.