If you have a light on the ceiling with un-switched 12 volts, fish a wire across the ceiling space. If the light is on the wall, you might be able to fish a new wire up the wall and across the ceiling - sometimes there is a hole in an interior wall above a wall light that you can get new wire into.
Otherwise, with some patience, it is possible to fish wire through a ceiling space to a 12 volt source by pulling down lights, speakers, AC trim, etc. for access points. You can always enlarge the hole behind a ceiling light for easier access. You should be able to find a 12 volt source somewhere like at the TV co-ax/amp. plate or radio, or perhaps from wherever the converter panel is and then up a nearby interior wall cavity.
We have a Maxxair fan in our bathroom and love it. Works great when sleeping in the bedroom. We open a bedroom window at night and run the bathroom fan. Cools better than having our bedroom fan running and sucking warm air into the bedroom from the rest of the trailer.
