The stuff in the track is called beltline. We had it in our 1979 but was dry-rotted and looked like blue cork. We took it out to polish and I have already ordered and gotten replacement. The replacement is like a pliable plastic (almost rubber). It is a little expensive but looks really good where I have already replaced in the A/C cover. I think we spent $125 for both the lower, wider beltline that wraps the entire camper and the thin upper beltline that goes around almost twice. We have a 31' Sovereign
as well. We got ours from online. There are a number of places that carry it other than Airstream and they are cheaper.