Trailer wiring issues

Hello again folks! Here's my problem: when dragging my bumper pull down the road, sometimes I lose my trailer brakes. This happened with my old truck and my new one. I took the truck to the shop the other day for some unrelated matters and had them look over it. They said they could not get it to duplicate, so it must be the camper (which makes sense because it did it on the old truck too). I don't even know where to begin to try to track down the problem. Any help?

Thanks in advance