It sure sounds like that you have isolated the problem to be the trailer wiring to the affected bulb. If the bottom of your trailer is accessible you might be able to trace the wire and find the break. If the bottom of your trailer is covered with Coroplast, you could open it up, but that is no small task. Somehow you gotta find the break or run a new wire. I think if I had this issue I'd opt to run a new wire, and to run it from the last known good point to the affected bulb. How to get the wire there would be the question for me. You might be able to get the wire to the rear by zip tying the replacement wire to components under your trailer.
If that is possible, you gotta get it up to the taillight. My trailer is hard-sided. Installing 6" of new wiring for a new license plate holder and light was a major hassle - aluminum framing was in the way. I could connect to the old wire that needed to be extended by removing the taillight; I then drilled a hole into the interior of the trailer and routed the wire inside and then back out to the new license plate holder. The interior routing of the wire I hid with a standard switch plate cover. All of which is to say that the easiest way to route the new wire may be on the inside in some plastic conduit cover.
Jayco's schematics of the rear wall of my trailer showed some 1" by 1/2" square plastic tubing that is used for wiring conduit. Such a Jayco drawing of your rear wall might be helpful.
Also: for my trailer, the taillight wiring runs from the front to the right taillight to the left taillight inside a plastic conduit in the wall. Another suggestion would be to remove the right taillight and see what you can find out; maybe the break is accessible there.
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
2011 Jayco 28.5RLS; 2008 Chevy Duramax; Pullrite Superglide Hitch