EXCEPTIONS TO CDL REQUIREMENTS:
From WI_343.055
http://www.hhrvresource.com/Images/P...%20343_055.pdf
Recreational vehicle operators. The operator of the commercial motor vehicle is a person operating a motor home, or a vehicle towing a 5th-wheel recreational vehicle or single-unit recreational vehicle and the vehicle or combination, including both units of a combination towing vehicle and the 5th-wheel recreational vehicle or recreational vehicle, is both operated and controlled by the person and is transporting only members of the person's family, guests or their personal property. This paragraph does not apply to any transportation for hire or the transportation of any property connected to a commercial activity. In this paragraph, "controlled" means leased or owned.
Looks to me like any truck in any condition with no improvements, even dual axle units, can be registered as an RV and driven with just a Wisconsin class D license.