I know when I first started driving in 1992 I was told it was illegal. Then a couple of years ago I was listening to an interview on a trucking radio show with Ray LaHood the Transportation Secretary and he was asked point blank if it was legal or illegal and he said it was legal according to federal law to carry a firearm in a commercial vehicle but the sticking points come when passing through the various states, counties and cities. That's why the vast majority of companies forbid it. Every company I have ever worked for, or leased to forbid it.
When I was driving a truck, it was against the law across the nation.
Maybe its changed now.