Welcome to the forum
Little loop thingy? By chance do you mean the butt connector that is crimped onto the wire and then held onto the battery with a nut?
If it was the POSITIVE wire, was the trailer plugged into shore power as he fixed it? If so he may have accidentally touched the positive to ground, and blown the “reverse polarity” fuses.
Look at your fuse panel for 2 40A fuses to the left of the row of fuses. I bet they are no good.
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2012 Copper Canyon 273FWRET being towed by a 1994 Ford F350 CC,LB,Dually diesel.
Airlift 5000 bags, Prodigy brake control, 5 gauges on the pillar.Used to tow a '97 Jayco 323RKS.
Now an RVIA registered tech. Retired from Law enforcement in 2008 after 25+ yrs.