Castle Rock Mackinac Trail Campground, St. Ignace Michigan

That's over by the casino. Never stayed but they have a nice location. Not many choices for St. ignace.

The state park is ok but suffers from a lot of noise from the bridge. Semis Jake breaking 24x7 and wild hogs clearing their pipes. Its otherwise so quiet that those sounds really stand out.

Just FYI, from your wording you have Sleeping Bear lumped in with UP destinations. It's a decent drive from St. Ignace to the dunes, at least a few hours, and across the toll bridge.