I'm not sure about the Airstream Pinnacle in particular, but many RV's of that vintage will not be widebody models, and none of them will have slides. Those features are certainly not required for fulltiming, but it may get awfully cramped without some extra interior space.
As others have mentioned (and floorplan/ interior design aside), condition and past maintenance will determine the potential usefulness of any 25-year-old RV. Unfortunately, many rigs that age spent a lot of time sitting still (driveway, storage lot, barn, field, etc.) and that treatment generally does not bode well for long term roadworthiness. The money that would need to be spent to refurb such a unit is often well above what the RV is even worth.
I'll move this to the Fulltiming area and let some of those folks chime in.