My vote is to replace the trailer wiring. Make sure the new wiring is installed with plenty of support and protected from wear and the sun as much as possible.
I built a trailer many years ago that worked fine with all the lights behaving until it began blowing a fuse occasionally. I have several trailers so it was easy to determine that this was a trailer problem. Fuses usually would blow when on the road in wet conditions. On my trip across Canada I began loosing rear lights, first one side and then the other. I bought some wire in Canada and ran a wire down the side along the outside for a new ground. I ended with a wire down each side to complete the trip. Shortly after that It began blowing fuses for the trailer lights. My Silverado pickup has separate wiring for it's tail lights and the trailer's circuit from the front of the pickup. I could not find the short since much of the wiring is inside the box tubing so I pulled out all the old wiring and pulled in a multi-conductor cable with an exterior sheath and enough wires for everything including backup lights that I added. Now all is happy and the wiring is an improvement from my original (maybe 30 year old) original job.
Sometimes a short can be intermittent getting worse in wet conditions due to the moisture conducting enough electricity to make the difference.
Remember, current doesn't only follow the path of the least resistance, it follows all paths and they are additive. One of Kirchhoff's laws.
