I don't have anything to back this up, but I recall from conversations in the past that it's not necessary for the towed vehicle to be tagged or insured.
I see people flat towing beat up cars/trucks all the time on 81 that don't appear to be registered. Not sure if it is legal or not though. I know it is in the state laws that are easy to find using google.
I talked with the Sheriffs department here about 2 years ago with the same question when I went up to buy a truck. They said if the vehicle has any wheels touching the roads it needed to be covered by insurance. I am not sure if laws have changed since then, or how loosely they do enforce it but the police department can answer your questions with up to date info and let you know all that stuff.