The thread posted my RISC777 pretty well sums it up, but NO, it is not against the law to carry in a commercial vehicle.
Now, you'll want to make sure (if you care) that your company doesn't have a policy against you carrying a firearm, and you'll also want to make sure you are following all fed/state/local laws while carrying.
It is a common misconception amongst commercial driver (mainly OTR drivers) that there is some Federal Law that prevents you/us from carrying a firearm while on duty. There is no such law.
With that being said, carrying a firearm while driving a truck OTR is probably not a good idea (if you care about the law) for many drivers, because chances are, you'll end up somehwere like Cali, Illinois, New York, etc. that won't allow you to carry even if you do have a valid HCP for your home state.
I drive in TN and KY only, and my company has no rules against carrying a firearm, so I am able to do as I please. Have done so for the last 6 years with no problems, and I've been stopped in the weigh stations on many occasions. Never any trouble there either.