I know dot laws are federal and I also know the moment they set foot into a (box) trailer it switches from looking to searching and that requires a warrant.
That's correct, they cannot "search" the load without a warrant (unless you are placarded for dangerous cargo). They "can" make you open the doors to inspect.
I see U-Haul trucks pulled over all the time, in multiple states, the reason is some small businesses will use them to transport their product without putting anything on the door etc. I see Penske trucks, Ryder trucks etc as well.
Federal law allows a DOT cop to pull you over if you're driving down the road with a ladder in the back of your truck. They think you might be going somewhere to perform work for someone for pay.
It's a nasty business dealing with them. When I started driving trucks 35 years ago it was nothing like it is now.