I'm having trouble visualizing where the hump is (and how big). A picture with some scribbles might help. You might try using a stud finder to determine whether the hump is coincident with a frame member. This might help you to decide whether it is evidence of a bent rail, or just plywood that is swelling with moisture. Speaking of which, you can get a cheap moisture sensor that is used in carpentry and stab it into the bulge to see if the moisture content is high relative to non-humpy areas of the trailer.