This topic comes up on not just this forum but others as well. The answer is that there are no laws preventing a "commercial driver" from having a loaded pistol in his Semi as long as he/she are in compliance with State law (Local too but most States now preempt Local for firearm regulation).
The issues presented to most Commercial Truckers is the prohibition of firearms on the various facilities they service. No entry to Federal Installations allowed with a firearm in the vehicle. Many private companies do so as well. Their property, their rules, sometimes even to inspecting vehicles entering or departing.
Owner/Operators often "lease" their rigs to companies to exclusively haul their products. If that company has rules, and the lease requires that the Owner/Operator adhere to them, no gun if prohibited in the rule book.
It's not against any law, it can just be inconvenient for those truckers that enter and exit prohibited areas regularly, Government or Private, so they just leave them home.