A very common problem, which I've posted about before is that some brands of RV trailer cords have a plug on them that isn't manufactured to the exact specs / industry standard, therefore they fit a bit loose in the OEM truck plug, which is usually a POLLAK brand plug.
I had the same problem, and a cargo trailer dealer I frequent told me about it so I changed the pigtail on my enclosed trailer, and that solved the problem. THe trailer plug fits the truck receptacle nice and snug now. That was 2 years ago and I haven't received the "check trailer wiring" on the DIC again.