If its good to the plug, and no lights work, it's a trailer ground problem as you've discovered.
Does the harness ground the frame, and the lights off the frame, or are the lights wired direct to the ground wire?
If it goes to the frame, it's between the plug and the frame. If not, it's between the plug and the first light
. Welcome to trailer wiring. If there are any scotch locks in the wiring, rip them out now and replace them with proper crimps and adhesive heat shrink before its too late and you have to replace the wiring.