Jim,
My 59 Ambassador International also has a vent fan that runs on AC or DC. In
DC mode it operates on 12v DC. There is a small transformer located next to
the fan motor which converts 110 v AC into 18v AC which also operates the
fan. There is a three way switch with AC DC designations. The fan for the
furnace also operates this way. Probably "state of the art" in a 1959 travel
trailer, but it is an Airstream after all.
Colin