Park at the golf course immediately across the street from Michigan Stadium.
From I-94, at exit 175 (Saline Rd) go north. The name of the street changes to Main Street. Go all the way to Stadium Blvd., make a right. Pull into the golf course. The golf course parks vehicles everywhere on the course except for the greens. One year I parked in the middle of a fairway five feet from a green. A beautiful setting to tailgate. The only drawback is that if you have females in your group, the port-a-potties are few and far between. You can enter the golf course from Main Street but you will have a longer walk.
Overnight RV parking is on Main Street on the west side of the golf course at Pioneer High School.