Ok, so I've been working my way through Season 1 (I'm on Episode 11), and I've noticed a few things about the truck.
Sonny has referred to the engine as a "903 diesel", and in several of the episodes you can clearly see the "903" on the valve covers & the turbo; so that leads me to conclude that the truck has a VT903.
In Episode 10, Will is concerned about driving a logging truck with a 5-4 twin stick. When Sonny tells him about the transmission, Will replies, "What? No buttons?". I'm assuming that he is talking about the transmission on Sonny's truck, but I'm not sure what he's talking about. Is he saying that Sonny's truck has an automatic transmission, or that it has an air assisted manual transmission?
I also noticed that the badging on the truck (the hood ornament & the "KENWORTH" badges on the side of the hood & sleeper are gold. This leads me to believe that the truck they are "driving around" is a 1973.
The interior looks to be a tan VIT interior, but I cannot tell what color the buttons on the back wall & doors are. In some shots the look green (to match the exterior? ), and in other shots they appear to be brown-ish. Does anyone know what color they really might be?
I have to admit, it's a pretty cool show. Never did see it back in the 1970s, and I actually hear about it from a 1984 AutoWorld catalog where there was an article about "correcting" the AMT "Movin' On" Kenworth.