1st off, welcome to the U-Haul
camper community! Most people who come to own one of the CT's or VT's really love them.
Is the outer skin really loose from the interior pieces, or does it simply look weird to you? I think they all came with that little strip of gray carpet covering the joint where the 2 halves of the trailer come together, and that's fallen away--leaving glue residue behind. Which admittedly looks odd.
IF that's all that's happened you can re-glue it in place over the joint, or glue some other trim over it.
[avoid silicone caulks/glues if you can--they're such a pain to clean up later, if you ever have to do other work in the same spot
If the inner & outer layers really have come unglued from each other, and there's movement in the pieces around that joint, that's another whole deal of course.
Best wishes with your project, in any event.......