Still, no one can say where I can read about it. I know different states have different rules but what do the feds say about Commercial Vehicles?
What do you want to read? The answer for guns in CMV's has been correctly answered here several times. The feds don't have anything for you to read because they have nothing to say about it. Since it's a right they're only going to write about it if there is a restriction. They're not going to write to tell you what you already have a right to do. It's not an issue of asking permission for a privilege. Are they going to say you can also chew gum in a CMV? No, because you already have the right to chew gum. So they're only going to write about it if they're putting restrictions on chewing gum.
Some resources to help anyone reading this learn their rights or refresh their memories. I'm not making any accusations. Add more excellent sources in the comments below if you like.
Understand the risk of talking to police.
Learn the Constitution.
Also check out the Flex Your Rights series
There are many examples of police interactions you can view on YouTube. You can see the pros and cons of different actions. Sometimes the cops are professional and sometimes they're not. Sometimes the people in the interaction are smart and reasonable and sometimes they're not. Sometimes they follow the law and sometimes they don't. And, of course, on the tv show Cops you can see people get arrested left and right because they didn't know their rights or how to use them. Usually they would have been fine if they would have kept their mouth shut. Instead they talk and give consent. Even when they know there are drugs in the car.