The Hiawatha National Forest exists in two different units, one in central UP and one in Eastern UP. NFs allow boondocking, especially along old woods roads. They also have nice campgrounds w/o hookups. I suggest you call the Ranger Stations at Munising and Sault St Marie/St. Ignace. Also, consider Tahquamenon Falls State Park. Very nice, but of course not free. The Ottawa NF is in the Western UP and also allows boondocking and also has campgrounds w/o hookups.
Huron-Manistee NF in the lower peninsula also offers camping and parts of the NF are adjacent to Lake Huron. Call the Huron Shores Ranger Station for info.
