I guess you don't know about
Rodriguez vs U.S..When they find out you don't need no stinking plate on your trailer they can no longer legally detain you. The stop can no longer extend beyond the time that it takes how to write a ticket. If they don't have a drug sniffing dog with them they can't legally detain you anymore to wait for a drug sniffing dog to arrive. They cannot legally subject you to a retaliatory "roadside safety inspection" either.
Know your rights and stand up for them. Not standing up for them is why they get away with it.