[rvcycleguy]

I'd call it a reinforced corner moulding. Looks like stress crack. The campers roof sections tend to have a lot of movement while being towed. I'd recommend using a "L" shape metal bracket to reinforce the corner so it does not crack event more. I've seen a L bracket with and without predrilled holes. You may get lucky and the predrilled type just may be in the right spot as the screws that fasten the moulding pictured. I'd suggest placing a weather type caulk in the crack before the bracket.

It does cover the corner and I wouldn't remove it. It may not go back on flush as the factory applied part was installed. It may also be a heat issue meaning the camper may have been stored outdoors, uncovered, and the corner plastic cracked.

Rv