Hello Ralf
Erich here in Ontario California
Used to work on GMC motorhomes A few years back. Still have 2 left in my shop yard. Both of them were stored under cover but not in a closed garage. both are California Coaches and have not rust issues! I have also a spare engine. Although it's a 455 not a 405. Either engine are interchangeable in these coaches. 73's to 75's have 455 Olds installed. 76 to 78 had 403 Olds engine.
Both coaches have to be redone inside and out! Got windshields and other parts too that go with these Coaches.
all could be your for around 20K's.
Regards, Erich @
fischergmc@earthlink.net