The hard fiberglass spare tire cover used on the Airstream motorhomes will not work on the Argosy motorhomes. On the airstream motoromes the rear of the coach is cut open using up space in the rear credenza. Its like a small pocket cut into the back of the coach that the tire slips into and then the cover fits over the opening. You would have to modify the back of your Argosy to make this work.
Our Argosy had a spare tire mount that plugged into the rear hitch receiver. I don't like it and plan on doing something different, just don't know what yet. I've see a lot of Argosy's and they all seem to be different in how the spare tire is mounted.
Bottom line is I don't think you'll be happy with the trying to use a spare tire cover from a later Airstream motorhome.