Could be that there was an air conditioner over the vent space. If so, the wire will be found at or very near the converter. If there are switches by the converter the switch will be for AC shore power, flipped the other way, AC generator. The wire at the vent opening may be tied to the switch. If there is a type of fantastic fan (12 volt motor) the previous owner may have run the existing 120 volt lines to the 12 volt panel rather than trying to fish new wires to the 12 volt appliance? In any case I'm sure the wire runs across the ceiling and down the wall to the converter area in the shortest distance possible.
Hope that helps.