Anywhere on the west coast of MI, Ludington, Muskegon, South Haven it will be nearly impossible to get a reservation at a State park on a weekend unless you make it exactly 6 months in advance (the second the reservation system opens). Any other areas of MI it will be a lot easier to get reservations except for Holiday Weekends/Weeks. Not knowing where in MI you are located, my personal preferences are the UP. The UP is like the best of the best for me. If you were camping in the UP you could probably wing it, other places in MI it might not work out.
I've also camped in PA, NY, TN, OH, WI, and many other places and there are a lot of spectacular sites to see in this country, don't be afraid to go places. Having friends that camp and like to travel also makes trips that much more enjoyable.