Double towing in Michigan is legal but requires towing behind a 5th wheel trailer. Double towing behind a Travel Trailer (TT/bumper pull) is not allowed.
Nearly half of the Michigan State Parks have very good marked trails for hiking and lots of forests and also are on rivers or lakes. Beside those already mentioned here's a couple more with serious hiking trails: Hartwick Pines near Grayling (9,700 acres, Tahquamenon State Park - eastern U.P.(huge -50,000 acres), The longest S.P. trail is Fred Meijer White Pine Trail State Park that goes 92 miles from Grand Rapids to Cadillac (old RR bed - about half is paved and the rest is gravel or packed cinders.
Then there's the huge Sleeping Bear Dunes National Lakeshore N.P. with a gorgeous CG at Platte River and more trails than you can do in a month right from your CG site. Over the dunes and/or thru the valleys and thick forests with lakes, rivers, and streams all over the place. Boating and/or fishing in a different lake every day for a month stay. Even go over to the 2 big islands (Manitou's) for more and more hiking until you drop! Virgin forests of still growing HUGE cedar trees that had started growing before Columbus crossed the Atlantic on South Manitou Island. Trees over 500 years old!
The to top it off, there's over a hundred State and National forest CG's that have untold miles of serious hiking trails right from your site. Most sites have no kookups.
So many choices and so little time!
Camping, boating, fishing, hiking, and hunting is king in Michigan.