Most likely your beltline molding is the plastic insert type that is available from any Airstream Dealer, or Inland RV and others. Just replace it all, it comes right out, and snaps right in. If it has little riveted clips at the end, you can drill them out easily with an 1/8th inch drill and replace with a normal 1/8 th inch Pop rivet, probably 3/8 grip range, which you can get at any hardware store, along with a rivet gun. Have them show you how to use it, if you are not sure, and do not have anyone to help you out.
When you replace the molding, don't stretch it, leave it loose so it can expand and contract with temperature.
Welcome to the wonderful world of RV maintenance. Even Airstreams need it. A lot of it.