Great info!
So I can probably assume the 23' (I stepped off 25' bumper to bumper) 1977 GMC Jimmy Mini I'm looking at is a G30 third generation van cab (saw the same thing on an X-Files Ambulance rerun just last night :) ) on a rail frame chassis rated at 12,300 GVWR, and that it's NOT likely a P30, which was apparently a larger frame chassis, not a later frame chassis (as I said earlier about it maybe being a fourth generation).
Thanks so much!
Still haven't gone back. Will do that probably next few days between weather and other chores.