You were referring to a U-Haul truck. A DOT cop can make you open the back door, he would need a warrant to search it, but not to look. It isn't search and seize, it's just inspecting the load.
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.
Oh I went back and read my previous post I get how that sounds I was referring to how the people handle the searching not to the laws themselves. My fault.