My comments are based on my '67 Caravelle and thus will assume your '64 is similar. Let's start by assuming the fans and lighting are on separate 12v
circuits/wiring all originating from the "Uni-Volt" box. The Uni-Volt gets 12v
(& 19v for battery charging) from a simple transformer.
Exterior light gets power from the lighting circuit. Difference is that it is feed by 12+ and grounds to the chassis. All other lights are grounded back at the Uni-volt. On my Caravel
, the exterior light is feed from the nearest wall sconce (12v positive leg only) fixture through a conventional wiring device (switch).
On my Caravel
, all the fans are on one 12v
circuit (i.e. vent fan and galley fan). load and ground originate from the Uni-Volt.
It could very well be that a mouse chewed the wires, but start at the Uni-volt (or whatever your trailer may now have) and look for voltage. Go to each fixture/fan and see where you loose voltage. The wiring is all daisy-chained from fixture to fixture.