Jim,
I checked the 05 Monterey prints in Coach Assist and there are no AC boxes shown in the ceiling. I think you are right re 12v for fan. If junction box beneath plate has a wood frame around it, fan is probably the intended appliance. Is line hot now? if yes you'll be able to use remote for control. (I'd still locate source by pulling fuses, etc.) If no, you'll need to find source or check wall in bedroom for intended switch location and hot 12v line. Other possible use for the 12v line you located could be lights, but in past they used white wires for them.
Have fun.
Steve