My boat trailer had a taillight out basically the second time I took her out and I thought the bad connection was on the truck side. I refused to think there was anything wrong with the brand new trailer and its connections.
Even after testing with my meter, manually putting 12v to each pin for testing, bending the mating tabs in the connectors, the wife complaining about helping me, I said screw it and cut the existing connector off the trailer and putting a new one on. Hasn't failed since
Electrical gremlins are no fun, especially wiring that looks like it should be so robust it "couldn't possibly be the problem." You can never really tell what's happening under the sheath
It might take some extra ppl to help, but try giving the connection a good wiggle while everything is running and see if it happens again. The new connector I installed on the trailer side had a much tighter fit than the original one did