oilspot

I agree with happytrails.  

I observed (and fussed) about warpage (or bubbling) in the walls, but eventually found out it is just the way the trailer is built.

The walls must bulge out slightly around each pocket stop to accomodate the extra hardware and reinforcing plates.

You note doesn't say much about location, but I would be concerned if the bubbling is on the roof or floor, but not around the support arms.