Be sure you spend at least a day and night at the Aune Osborne Campground in Sault Ste Marie. You can watch the big ships/barges move up and down the St Marys River right next to the campground. Be sure to visit the Soo Lock and Dam visitor's center too! You can stand in the observation area within a few feet of the big ships and watch them drop them 24 feet in 30 minutes to move from Lake Michigan to Lake Huron via the St. Marys River.
We've stayed in excellent campgrounds at Tawas Point State Park in East Tawas, Hoeft State Park near Rogers, Tahquamenon Falls State Park (great brewery/restaurant near the Upper Falls), Munising Tourist Park Campground is a great place to stay while taking the boat tour of Pictured Rocks or a glass bottom boat to see the Lake Superior shipwrecks. You should also visit Great Lakes Shipwreck Museum at Whitefish Point on Lake Superior. It's very interesting and a pretty area. Lots to see and do in the Upper Peninsula!
x2 on all of this. We also enjoyed the Wandering wheels CG in Munising - woodsy but room for most rigs, it seemed. Petoskey KOA is pretty resort-like but made a nice base camp for Sleeping Bear Dunes.
On the way back home, through Wisconsin, consider some of their state parks, especially on weekdays. Particularly pretty that time of year would be Council Grounds, and Point Beach SF is one of our all-time favorites. Plenty more in that department. We stayed at the KOA in Eagle River & really enjoyed it - lots of trees, and without that parking-lot feel that many private CGs have. Gorgeous there that time of year too.
Should you get up to the "rabbit ear," McLain (sp?) SP is great!