There is no federal law concerning commercial truck drivers and firearms. If you have the proper Permits/License and can legally carry in the state you are driving in or that state allows the carrying of firearms without a permit you are legal.
A company can have a Rule that states you cannot carry in their vehicles. That is just a company rule and all they can do if you do carry and are caught by them is fire you. You are not breaking a law just a company policy. It is very likely a driver would be terminated if carrying a firearm against company policy, but they would not be subject to criminal law as long as they are in compliance with carry laws in the areas they are traveling through.