No idea what it might have been, but the white is a ground wire and the yellow is 12v
. You should see a yellow wire coming from one of the 12v
circuit breakers. In most cases, a circuit services a number of things in the same general area of the trailer.
Did you check with a meter or light to see whether the circuit is hot? Someone may have disconnected it elsewhere.
Also, in my trailer, there are whole connectors that are unused because the trailer was never initially wired according to the wiring diagram. It's possible the wires go nowhere.