well, the GMC I drove was the W7. The biggest of the sub 26,000 pound Isuzu based trucks. It would hold three people (feel sorry for the guy in the centre seat) and would get 11mpg.
that is.. 11mpg empty or 11mpg full to the maximum Gross weight. The Nissan, Mitsus, and HDs I also drove on occasion would usually hover around 9mpg.
The Bigger International Drove was a totally different matter.
The Cab on the W7 was pretty much the same as the other Isuzus except for the smallest trucks. With the third seat down, I could lay across all seats and JUST touch both door panals. I am 5'11", so I could always catch a good nap.
As for the truck inspections. You are at their mercey. As long as you have commerical tags, you HAVE to pull off of the highway when directed and you WILL get hasseled by the local mobile units. Keep the truck neat and clean, keep your logs in order and obey the same rules the OTR truckers do in regards to hours driven and hours resting and you will be fine. Inspectors like neat trucks and neat logs.
If you can handle a stick, I recommend the manual transmission trucks. The Autos are ok, but to get the most control in both slowing and in bad conditions, a manual is a good thing.
As for problems with the W7. I put the first 200,000 miles on that truck. I started blowing fuel lines at 120,000 and I think 5 of the 6 I blew were due to bad lines as it was always the same line that blew out.
Also had trouble with the 'spitter valve" at the bottom of the brake airtanks. I could spend $75 for the part plus labor to replace it.. or I could take off of the tanke, undo four bolts, clean the diaphram ( 4 inches diametre) with a rag, and put it back together and reinstall. It would get gummed up with oil from the aircompressor.
The Turbos started to go after I stopped driving that truck and letting it cool down (oil cooled turbos) before shutting down.
Be sure to bleed down and drain the airtanks at LEAST once a week. One "new" truck I did it had it's tanks 3/4 filled with water. The braking got a LOT better after I drained them.
best advice I can give any potential driver.. find out how tall your truck is.. add four inches and put stick on letters where they cannot be missed reminding you how tall your truck is.