I am a transplant from Michigan, so I know the area pretty well and am very fond of the state. That said, do not miss Minnesota's Lake Superior shoreline! From Grand Marais to Two Harbors, you will find some fabulous state parks and views. The high escarpment which parallels the shoreline results in many nice waterfalls within a short stroll. At Grand Marais, walk out to Artist's Point and then have a great pizza at "Sven and Ole's". Take a drive up the Gunflint Trail and enjoy the quiet wilderness (with several NF CGs). Cascade River, Temperance River, Tettegouche, and Gooseberry Falls are all good spots to stop and see the sights (and camp, if you can find a space). Be sure to see the Split Rock Lighthouse; it's very picturesque. Unhook and take your truck to Palisade Head, high above the lake with a cliff that goes straight down (one can often find rappellers here).
When you head into Michigan, spend some time in the western U.P. near the shoreline there. Porcupine Mountains SP west unit has a river with 3 small waterfalls in succession, and you can walk on paths all along the river. The east unit features a high overlook above Lake of the Clouds. CGs are available at either unit, plus there's a nice municipal CG right on the lakeshore in nearby Ontonagon.
Then head northeast into the Keweenaw Peninsula and try to get a campsite in McLain SP, another CG with Superior views as well as a good ice cream shop at the south end of the park. Be sure to drive your truck north along M-26 between Eagle River and Copper Harbor for some great shoreline scenery, and visit Fort Wilkins SP if you like historical stuff. Find a small store and buy some thimbleberry jam.
From there, I think the next "don't miss" item is a boat ride along the Pictured Rocks National Lakeshore; tickets are pricey, but worth it when you add up the views plus the humorous comments and stories you'll hear from the captain. Try to take the sunset cruise on a clear, sunny afternoon for best light on the cliffs.
Farther east, Grand Marais MI has a nice CG (Woodland) on the water, and there's an interesting high sand dune just west of town. Then you're off to Tahquamenon Falls. And north of that, at Paradise, you might like to see the lighthouse with its modest museum. Near the Soo, my daughter tells me that Brimley SP's CG is very nice (I haven't been there personally), and of course the Soo Locks might interest you.
Be sure to order some pasties (meat pies) for a meal while in the U.P. You can probably get some fresh fish around Munising, too.
Mackinaw City is home to a great many small shops for tourists. Drop your trailer at Mill Creek and drive west a bit; there's a lighthouse just a couple miles west of town, and another one at Wilderness SP (sites there are probably too small for your rig, unfortunately). There's an historic fort at Mackinaw City and another on Mackinac Island. Take bikes on the ferry to the island, if you have 'em (or rent some on the island if you like); my favorite thing there is to ride all the way around the island. No cars!
