The windshields, cockpit window glass, side window glass and rear windows are all the same. The differences were in how the frames were trimmed out with most of those changes occurring on the side windows.
Some of the Classics used push out trailer windows in the bathrooms. I'm not sure what models and years did this but I know 84 and 86 had the push outs.
The floor structure from 74-76 is different than 77 and later. I also think somewhere in the early 80s they made another floor change but I'm not certain of that.
I believe the number of ribs increased as the years went by. My Argosy has minimal ribs whereas the 345 I dismantled seemed to have considerably more per running foot.
The quality of the trimmings inside also improved year after year with the Argosies being the "cheapest".
Argosies were manufactured in Versailles Ohio and the facility was shutdown in 1980 (I think that was the year) and production was moved to Jackson Center Ohio.
The entry door hinges are actually trailer hinges turned upside down. The hinges are labeled Top and Bottom but on the motorhomes the Bottom goes on top and vice versa. The doors themselves are the same as trailer doors.
The front grilles on the 74-76 Argosies were completely different than the 77-early 90s models. Same for the front and rear bumpers for 74-76.
I'm sure there are a lot more differences, these are just the ones that come to mind at the moment.
Air forums # 1674
1974 20' Argosy Motor Home
1975 24' Argosy Motor Home
1974 31' Excella trailer (parting out, as of 4/1/2015 I have wheels & windows left to sell)