As you're asking this in the concealed carry discussion section there's 2 things that might prevent you carrying in a CMV.
The first one is whether your permit is recognized in whatever State you are in (for example, your permits might be good while in OH and PA but if you have to go to MD or NY what will you do), the second possible barrier is the company policy of whoever you work for or are leased on to.
However, there are no federal laws specifically preventing a firearm being carried by the driver of a CMV.