Most likely a ground wire.
The standard in the old rigs was Black = Positive and White = Negative. Because "back in the day", when they where still figuring out how to wire 12V
trailers, the standard in house/building wiring is Black = Hot & White = Neutral (which is fairly close to ground). This grounding threw me for a loop (pun intended, sorry) when wiring my '63.
I dont know if or when new trailers switched over to Red = Positive, Black = Negative, but its noted that the plug at the TV uses black/white.
Then again, some previos owner might have needed to replace a large gauge wire and white was the first color on the shelf at home depot.
Best best is to get an Ohm meter, or continuity tester and the schematic for the trailer to trace out where it ends.