Dan I did a quick check and an Airforums member that goes by the handle "TricoastalJoey" has or had a 1976 28' Argosy like yours. He was not very active on the forums and his last post was in December of 2006.
Ben (bpg_austin) has a 78 28' Argosy and is currently active on the forum. You might send Ben a PM asking for help.
Oh yeah, almost forgot. Martin (martin300662) has a donor 28' Argosy that will soon be heading to the scrap yard. He likely won't be able to remove the parts himself due to where the donor is parked but you might reach out to him to see if there are any options for getting the parts.
Air forums # 1674
1974 20' Argosy Motor Home
1974 31' Excella trailer (parting out, as of 4/1/2015 I have wheels, brake drums, windows & holding tanks left to sell)