If you are going to Mackinac Island, plan on a couple of days minimum. There's a lot to see and do. Just park your MH in the parking lot of the ferry service that takes you to the island. For a few bucks more a day, the ferry I used had a fenced in area for high value vehicles to park in. That's a lot cheaper than any campground in the area.
We stated at the Lilac Tree Hotel and loved it.
http://www.tripadvisor.com/Hotel_Review-g42423-d262585-Reviews-Lilac_Tree_Suites_Spa-Mackinac_Island_Mackinac_County_Upper_Peninsula_Michigan.html