From the Mackinaw City area (I hope you are planning to visit Mackinac Island!!) I'd also recommend Tahquamenon Falls SP - nice CG there, electric hookups available. Popular spot - you may want to look into reservations. The shipwreck museum at Whitefish Point is well worth a day's visit.
A good CG at the Soo (Sault Ste Marie) is Aune-Osborn RV park, a county-run park. Not much to look at, but it's clean and the view of the big freighters going RIGHT NEXT TO the CG is amazing. Be careful in this area using your cellphone - you may get routed to a tower on the Canadian side & charged international rates! It's worth spending a day there, the boat tour thru the locks quite a fun ride.
Farther west on the UP, there are several good state parks to camp at. We especially liked the smaller (west?) unit of Indian Lake SP - not too many sites with hookups, and only pit toilets, but beautifully wooded and quiet; and McLain SP on the Keweenau Peninsula, in the western UP, is wonderful - right on Lake Superior. (We camped in these areas in September, so no worries about whether they'd be open.)
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