I live in Milwaukee, and did a little research. Looks like there is a temporary permit you can get from WI (vehicle has to be currently titled in another state to quality). That allows you to bring a vehicle into the state legally without being registered here. (At least that's my understanding
banman is correct you DO NOT want to get pulled over here in a commercial vehicle without insurance, besides the listed possible side effects you could also loose the ability to hold a license here for 2-5 years.
Sehnsucht is also correct, WI CDL requirement is based on seating of the vehicle, not actual passengers.
Once you have removed the seats AND painted it something not considered yellow, you can register it as anything from an auto to a heavy truck. RV/Skoolie (we actually have a skoolie classification) has tighter requirements such as a bathroom, running water, a kitchen, etc.
Not to add to your list but do you know where you are parking it... you can't park anything that large on the street, and some neighborhoods do have rules about what can be parked in your driveway and how long.
Hope this helps with what must be a crazy planning experience