We watch to see how the RV parks are filling up and call ahead when it seems to be very busy, especially on weekends. With some state parks being closed and some private CG's going out of business because of less travel in recent years, you never know what is ahead. I don't know about Michigan and Wisconsin, but state parks and federal campgrounds out here are using a concessionaire to handle reservations at $9/reservation. Concessionaires are now running a lot of the federal campgrounds and have doubled prices. State parks keep increasing prices and charging extra for things like daily admission + camping fee in Colorado, so public campgrounds are getting very expensive. But a lot of small towns have campgrounds that are quite reasonable and have hookups although wifi may be extra. There are a lot of those in the Midwest.
I check RV Park Reviews :: Home
, Woodall's, AAA and the Forum to get an idea of what's up ahead. We don't like to make a lot reservations far ahead because we change out minds sometimes and stay longer somewhere or go somewhere we hadn't planned on. Also, we were told by a CG owner that people were traveling shorter distances and staying longer to save on fuel. Popular public CG's can be booked up months in advance although some have walk in (really drive in) spaces available if you get there at the right time, usually midday.
We usually travel in the shoulder seasons because it is quieter and campgrounds are less crowded, but even then you can run into anything. Not everyone can do that and has to travel when it seems everyone else is.
We've never ended up on the side of the road, but sometimes you have to act fast when you see campgrounds along the way filling up in early afternoon. Unlike a lot of RVers, we get up late, leave late and drive late, but no one answers the phone at a lot of private CG's after 5, so we start calling in mid-afternoon when things look tight.
We traveled up the east coast of Michigan, north coast of the UP and Wisconsin and then to Duluth (nice city campground there) and up to northern Minnesota several years ago—not quite your trip, but similar. Mackinack Is. was worth a ferry trip—no cars allowed on the island. We stayed at a private CG just south of Mackinaw City, maybe Mill City, which has some nice sites along the Lake Huron and a view of the bridge. It wasn't officially open yet—very early in the season—but they let us in and we had the place mostly to ourselves. Mackinaw City has a public CG too. Gigantic place (550 sites), but served our needs.