The driver of a Commerical Motor Vehicle can carry a firearm on board. You're concidered to be 'not in commerce' which simply means carrying a firearm in the cab is regulated by the state you're traveling through. That's the good news...
The bad news is... your subject to the laws: state, county, and municipal of the jurisdictions you happen to be traveling through. Which can be confusing no matter how much research you do. I tend to check:
Carry Concealed,
Concealed Carry, Reciprocity Maps, Concealed Weapons Permit, and the state website of a state I'll be entering.
In my case I carry everywhere I go. Either on my person (in the states that recognize my CCW) or locked up in accordance with USC 18, Chapter 44, Sec. 926(a) when I travel through a state that does not recognize my CCW.
Sec. 926(a) requires that your ammunition or firearm be locked up, but some states do require both. So, to be safe lock them up seperately as was previously suggested.
Company Policy for most major companies is no firearms. Although, some smaller companies do permit it. In my case I'm not subject to a company's policy since I'm not employed by a company. But be aware in some states if you're a company employee and your company prohibits firearms your CCW is void in a company vehicle.
Shippers/Receivers are hit and miss. Some prohibit firearms and some don't. In cases where they do prohibit firearms I lock them up. I don't go to military bases or prisons, and I do not cross into Canada!
Generally, I don't advertise that I carry or what I carry. It's no ones business but yours. But if you are stopped and you have a CCW on your person I recommend that you inform the officer that stops you. I realize it isn't always required, but I've found that if you're upfront about it they don't get worked up about it.
One final thing. Scales... If you're stopped at a scale do not carry into the scale house. In most states the scale house is treated the same as a police station, so its unlawful to carry into one.
.