I have a similar 71 GMC Custom Camper and the info above is correct. Seriously heavier springs and a big sway bar.
The one I have is pretty plain Jane though and doesn't have any real fancy options on it.
On these the extra gas tanks, Mirrors and what not were all dealer installed options and that is the reason you see these trucks with so many different style mirrors, rear bumpers and with or without the secondary gas tanks.
