Given the results you posted, the first thing I'd do is check another trailer to see if I get the same reading (then it's normal) or if I get different reading(probably an abnormal indication)....
Secondly, I'd contact the manufacturer of the moisture tester to ask them specifically what you discovered and whether it's a part of the limitations of the equipment.
Then, as a quick "do it the easy way", I'd do a "simple Google search". When I just did that, I found this:
https://www.nachi.org/forum/f18/mois...readings-4905/ Here's what that link said: "Yes, James, any RF moisture meter will give you false positives if it encounters metal under the surface including corner beads, screws and nails, Pipes and old metal plaster lath."