Are air guns allowed on commercial vehicles?

Sure. You can have a real gun, too. The bit about the supposed federal law prohibiting firearms in the CMV - entirely false. I'll bet you a week's paycheck you can't find this supposed law anywhere in the FMCSR or any FMVSS guidelines - because it does not exist. Laws for carrying a firearm in a commercial vehicle are the same as they are in any other vehicle, and these laws vary by state. Now if you're prone to delivering on military or government installation, or if you cross the border into Canada, I wouldn't recommend it.