Pulling new wire through the ceiling of an Airstream is almost out of the question because of the metal ribs and the fact that the wires go through those ribs.
Airstream generally runs the factory wiring down the center of the ceiling making it even harder to steal the old wire. The fan is probably the first thing on the circuit and thus you could not steal the 110 wire to the fan without losing the balance of that circuit.
One option is to find a small surface mounted wire channel and run the 12 volt
to the fan.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles