If your thinking about western Michigan in August, start making your reservations now, If you can still get some. Besides state parks or Private RV parks, check for county parks also. Getting sites during the week should be no problem, on the weekend it may be a crap shoot. I didn't search in western Michigan last week, but I was able to get a site for the weekend of July 29+30 at a state park and a site for the weekend of Aug 19+20 at a county park. both in Eastern Michigans thumb area. All state parks have some sites that are first come basis but not very many large enough to handle a big RV.
If your looking for full hookups, RV Parks are your best bet. Only a few state parks have full hookups but many of the county parks do.
Hope this helps, ShermanD