Loose wire somewhere. The connections might look right but might need to be redone.
So you're saying the brake light portion of that same lens is working, right? If so, it's the right turn signal lead from the truck that has a poor connection somewhere.
I had the same problem a few years back. I have TWO 7 pin trailer connectors on my truck. One at the bumper hitch and one in the truck bed for the fifth wheel hitch. My left turn signal didn't work on my 5er after 6 months of towing, but I was able to plug the 5er's trailer plug into the bumper hitch and that worked. I ended up going under the truck where the dealer's installer had tapped into the wiring and took each connector apart and redid them. Problem solved. It was tedious since the connections were taped and wrapped and hard to get to, but it worked! No problems since.
Check and make sure you have the right size buld should be a dual elementif thats not the problem make sure the ground wire has a clean connection. You may need to remove ground wire and clean terminal and buff the area around the screw hole with a wire brush or file try to get down to clean metal on all contacts. If that fails you'll need to trace the wire back to the plug for cracks in the insulation or broken wire. A good elelctrical tester can save ou a lot of time and frustation. Problem could be in the truck wiring terminal or camper wiring. Check these connections with a tester with camper plugged into the truck check to make sure you have power to the tuning signal if no power problem is either connector or truck, You can also purchase testers that plug into the truck that would check terminals. takes the guess work out of it. good luck wiring problems can be troublesome even with all the correct tools.
To test the bulb, take the left side and put it on the right side. If it fails the same, then the bulb is fine. It's either the fixture or the wiring.
Good idea on the tester.... I neglected to mention I used a meter to measure the signals coming from the truck at the connectors without the trailer attached to determine it was the wiring on the 7 pin connector in the truck bed not receiving the signal.
You do have to know the schematic, and be somewhat familiar with debugging using this method.