Gene-
Did the factory say if the embedded wiring had 'constant hot' wiring in it? If so, ask them what color to look for on the wire. Tapping off of that for the fan should have you working off of the battery disconnect switch, hence, desired result.
Otherwise, I would opt for the light that isn't switched at the door and see if you can run a snake above the ceiling cover and pull a wire through to attach the fan.
Maybe our friend Dario has an idea?
This is why I now have the factory install any items using electric I think I might want later. I learned from my Trail Lite.