Well, wanting to stay on one site for over 14 days eliminates the State Parks as they have a 14 day limit per stay. You'll most likely need one of the private CG for a several months stay. At least 25 of them in the GR area.
I live on Lake Michigan approx 32 miles west of Grand Rapids and go to GR often. Google lists the Grand Rapids close alone as having 19 CGs and that's about half of less of the total in the local GR area as I know many more that are not on the list. Grand Rapids metro area has 1,040,000 population and is a very nice large city and an excellent very clean modern downtown with tall buildings over 30+ stories high as so much to do in the area. Excellent shopping, eating/dining, entertainment unless you will be working 24/7.
Kalamazoo area has a lot of CG's also. A Google quick check showed more than 10. Kalamazoo metro population is approx 330,000. Good city but not as nice as GR. Roughly 50 miles from GR to Kalamazoo.
One huge CG/resort to check is Sandy Pines approx 6 miles west of Wayland, MI (approx 20 miles south of downtown GR). Over 800 acres and they offer seasonal sites. It huge with about everything you can imagine for attributes and has it's own large sand bottom very clean lake with beachsand beaches too. Anywhere in West Michigan will fill up fast so don't delay as RV'ing and camping is extremely popular.
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