was the very first year for the Univolt system, where the light bulbs, fan motor and pump motor all run off low voltage electricity whether switched to the "Battery" setting in the rear service center, when they all are powered by the battery, or switched to the "City" setting, when the trailer is plugged into 120 volts ac at the campground. This Univolt system allowed Airstream to use single voltage light fixtures, where both sockets in a light fixture are wired for the same voltage, instead of dual voltage light fixtures as in 1963
and earlier years, where one bulb is wired solely for 12 volts dc and the other bulb is wired solely for 120 volts ac. Conceptually, 1964
Airstreams provide twice as much light on any one setting.
Being the first year for Univolt, 1964
trailers are wired differently than 1963
and earlier trailers and differently than 1965
and later trailers, which may be why other's advice and experiences are not particularly helpful unless they also own a 1964 Airstream. The discussion below assumes your trailer wiring is original and hasn't been altered or changed by any previous owner.
For 1964 ONLY, when switched to "City" power in the rear service center, the Univolt system uses a AC transformer to drop the 120 volts ac coming in from the campground to 12 volts AC for the light bulbs on one circuit and to ~18.5 volts AC for the fan and water pump motor on a different circuit. The light bulbs don't care whether the 12 volts they get is AC current or DC current, they glow just as brightly on either.
The motors used in 1964 were designed to operate on both 18.5 volts ac and 12 volts dc. But today's dc motors, lkke the one in your Fantastic Fan, won't operate on 18.5 volts ac but will spin happily on 12 volts dc. That's why it runs fine when hooked to a battery but not when wired at your roof.
There is no rectifier (that changes ac to dc current) in this 1964 system, so when switched to "City" power in the rear service center, the wires for light bulbs should read 12 volts AC and the wires for motors should read 18-19 volts AC. If the Univolt - which for 1964 only is just an AC transformer that reduces AC voltage to 12 volts AC for the light circuit and to 18.5 volts AC for the motor circuit - is defective, you may get a reading of 120 volts AC on those wires instead of 12 or 18.5 volts ac.
When switched to "Battery" in the service center, those light bulb and motor wires should both read 12 volts dc on your VOM (Volt Ohm Meter). So with wholly original wiring, your Fantastic Fan installed in the roof vent should work when the trailer is switched to "Battery" power but should not work when the trailer is switched to "City" power.
To run a Fantastic Fan in your roof when on "City" power, you should rewire your trailer so it always has 12 volt
DC at the roof vent. To run a modern water pump, you also need to re-wire it to provide 12 volt
DC to the water pump instead of 18.5 volts AC. The 12 volts ac now at your light bulb sockets won't run the Fantastic Fan because its ac current instead of dc current, which the Fantastic Fan requires.
An original 1964 Unoivolt transformer - which weight pehaps 20 pounds - will not do this. So you should replace it with a modern combination rectifier and smart 3 stage battery charger, like the Intellipower 9100 or 9200 series or other brands mentioned in the Forums.
If the roof vent wires that read 18.5 volts ac when on "City" power do not read 12 volts dc when switched to "Battery" power, then the 12 volt
electrical system in your trailer is not working properly.
I'll stop here for now and let you determine how your trailer is presently wired and tell us what those readings are on both the "Battery" and "City" settings. Once you know that, then you can proceed to rewire it. I hope this helps you sort things out.