I have a little firsthand experience with traveling state to state and different laws. In the state of Wisconsin license plates are not required for small, non commercial trailers. However Illinois requires the license plates regardless of where you are from. My son with a Wisconsin registered truck towing a trailer with no license plates hit a deer soon after crossing the state line of illinois. State police showed up and his truck was drivable but he couldn't continue because he was threatened with the trailer and the two motorcycles in it being impounded.
After phone calls to his uncle, owner of the trailer, and me, in Illinois and owner of the bikes, he was allowed to proceed with a warning. We think the officer was just being a jerk to a 25 year old.