That's where we stay. It is a beautiful private park. 600-700 campites. You can see both Mackinaw Island (and the Grand Hotel) and the Bridge from the shore at the park. Ore freighters pass just this side of the Island. You can watch the ferrys moving back and forth, especially the rooster tail jet hydroplanes.
Terrific park, but no affliations that I know of. I need to check on that though this year. As you can see by their "Sold out" sites, they don't have a need to be drumming up business too much.
Here's Mill Creek Info:
Home:
http://www.campmackinaw.com/index.htm
Rates:
http://www.campmackinaw.com/pop-up_Rate.htm
Sold Out Sites (updated twice a week):
http://www.campmackinaw.com/sold_out_camp_sites.htm