See crude drawing below
. I installed this little latching relay in my Hot Water Heater, after 2 or 3 occasions of not turning it off when leaving the camp ground, only to find it heating water at home ( had I drained my system, I would have burned up the element). Cost is about 20 bucks, bought the relay off of ebay with mounting socket and mounted the entire thing next to the HW heater. I have the Hott Rod heater in mine, it is an after market mod, but this will work with any element as long as the relay contacts are sized to handle the current draw of the heater element. The way this works. Hook up at campground as usual, turn on your HW electrical switch and then hit the test on the relay. The relay will lock itself in and keep power supplied to the heater ciruitry. Now the neat part...when you unplug shore power the relay drops out and kills power to the element until it is manually reset after again applying power. No More burned up elements from forgetting about HW heater electrical switch.
If one wanted to, this could be wired to use a remote switch to do the reset. I preferred the manual method.