we have discussed this here several times.
There is nothing in the Federal Motor Carrier Safety Act that says you cannot.
Now, if you are looking for a law that says you CAN, you will have a problem- since laws generally say what you CANNOT do. Are there places you cannot take a gun IN a commercial vehicle? Sure. Do not take one into a Marine Port, Military base, gasoline refinery, explosives manufacturer, etc.
If someone tells you there is a law that says that you cannot, ask them for the law.
Want to double check? I will bet you a doughnut that your State Police has a Motor Carrier Safety Division. Talk to them.
What we have here is... failure- to communicate.