Having taken a second look at the diagram that grtharris posted, I think a fused wire from your house battery solenoid with a switch, connected to WHT 2 would work. While traveling leave it on all the time (the relay would be energized) , and control the electric thru your existing switch. The battery bay is right next to your heater.
Dan & Sharon & Kasey (Our Yorkie Puppy (12 Yrs Old) On the Road (2012 Journey 36M, 2006 Jeep Liberty)
USN-Ret ('65-'93) Fulltimers, Class of 2012