I am going through a brake problem my self. Like you said ask a fellow camper if they would connect to your unit, and you connect to theirs. This would help to find which unit has the problem. Check the brake controler fuse in the truck, making sure its tight. Check the under side of the trucks 7 pin make sure all wires are connected etc. I would then get under the trailer check all the wires going to the breakes for broken, loose, bare. Next step would be to open up the junction box where the pigtale is connected to the trailer. Make sure all connections are tight and clean. I would go so far as to pull the break a way switch and make sure it works. This would tell you if you have power going to the brakes.
If those checks to not fix the problem it would be time to visit a repair shop. Any trailer place would be able to trace the problem.
