The only big wires that I could think of are:
12V main buss wire.
Inverter wire
Brake wire. (Mine are very big wires)
Charging wires to the batteries.
Other than that I don't know of why they would have a huge 12V wire going to the back of the trailer? Could be some sore of a weird add on for some reason.
I do think your on the right track with that huge wire though. Look for arc marks on it or frame. A blind man should be able to see it where it arced.
I have seen a company that makes a 12V arc welder that are for sale. :E They really don't weld too bad.
