Do you have a wiring diagram for your rig??
In my case the switch for the electric water heater is 12 volt, it actually controls a relay that engages a 120 volt relay. The relay is in a box mounted behind the main circuit panel. Not easy to find but it is shown on the wiring diagram with a notation of location.
2002 Monaco Windsor 38 PKD Cummins ISC 350 8.3L
2005 Jeep Grand Cherokee w/5.7 Hemi