Firearms in 18 wheelers are not regulated by the Feds.
Truck drivers must obey the firearm laws of the State through which they are traveling.
I was an owner/operator for three years and have seen this topic discussed many times on trucking forums.
I've always hated stopping along I-70 right before the IL. state line to separate my pistol from the ammo, lock the pistol in one box and the ammo in another box.
Most of them had CC permits (and the ones I don’t know about might have).
Spent many a coffee talking about it. A lot of times the driver had been held up once before, and then decided to protect himself.