GM has one thing against them -- chassis types have moved to a new generation, starting with the T1N Sprinter. If people compare a rig on the 4500 chassis to a Transit, Sprinter, or even a ProMaster, they will see that its MPG will be in the 6-10 range while the competition is in the mid to upper teens.
The low MPG is understandable with the Fords because of their lifetime and the fact that the Transits/Sprinters/ProMasters would not support a 30+ foot vehicle. However, Chevy seems to be entering a market that would have been great in '05, but these days where people are very concerned about gas prices, they are behind the times for everything but the longer rigs which can't be built on the Euro-van chassis types.