Prior to 2007 I had towed them without a plate, although I understood they were considered implements of husbandry (farm equipment) which apparently was incorrect even then.
https://www.michigan.gov/sos/0,4670,7-127-48268-179824--F,00.html
http://www.treebuzz.com/forum/threads/michigan-specialized-mobile-equipment-registration.9638/
Also in MI operating with the wrong plate can be much worse than traveling without one. The vehicle in question can be impounded, the operator can be arrested, and the max punishment is 90 days in jail. It is misdemeanor, not just a civil infraction like many typical motor vehicle offenses. I can provide the references for the civil infraction for failure to display plates if requested. No plates is a better way to go in MI. My observation is that LEOs move into action pretty quick when they run a plate while following a vehicle and it doesn't match, Although it might be hard to tell if it was a trailer.
I'm glad to hear the OP made it. Navigating the varying state rules can be tricky. I had a friend that bought a Ford Bronco in OH. He called his insurance company and had a proof of insurance sent, and then printed it. He headed for home in MI with no plate, as would be allowed in MI directly after purchase. He made it 50 miles before he was pulled over and the vehicle was promptly impounded. Three trips to OH and $750 later, his Bronco was safely home in MI. The OH authorities were adamant that the only way to travel in OH is with a plate. Without one, you can buy a temporary tag at the BMV before you drive. Exactly the opposite of MI, and also surprisingly hard to look up.