The jumper is only to test to see if you have a faulty ground,
You fix it by chasing down where the fault is and fixing that. That could be in the connectors, the wiring, anywhere. I had to change the factory 7 pin connector on a 2 year old pickup last year because it was corroded internally and would not let the ground work. You changed the connector on the truck so you seem to be confident of that. I'm guess that since it failed on more that one vehicle your issue is in the trailer ground. Once you establish that is is a ground you just start working your way in one direction or the other until you find the faulty connection. Molded rubber or plastic devices can look perfect from the outside and be corroded beyond repair inside. Wires can be crushed to where they open internally and look just fine. You just have to chase it down.
You do the test to see if a clean wire, touched to clean spots on both trailer and truck frame, with everything else hooked up, completes the circuit.
I have 14ga wires with small alligator clips made up to do this with. I would not consider a jumper cable the best wire for this unless I was absolutely, 100% certain that the jumper cable was making a good connection. One test that gives you a false positive or negative can really throw you off in this stuff.
