Every ferry I've ever been on, here and abroad, allows you to drive your own vehicle(s) on to the vessel. Once you are directed to a spot to park, they will put out chocks to insure no rolling about. Most then require you to go to the passenger decks until you dock at the other end.
RVs are charged $5.95 per foot, one way.
If going from lower Michigan to Wisconsin, it's cheaper and faster than going through Chicago.