I'd take it to an RV dealer or manufacturer for repair. If your a "do it yourself" person, you could use Stainless steel sheetmetal screws or pop rivets. Just predrill holes to prevent fiberglass cracking. One seam is flush with side panel... use a good silicon clear chalking to prevent water getting in.
Clean everything with rubbing alcohol. You could also apply a small bead of chalking in the gap before pulling is shut and let the chalk squeez out some.
Mark and Maryellen (aka Mel) Kassab, Brevard, NC