I-69 takes you up to Lansing, then across through Flint to Port Huron, not all that far north of Detroit. Most of what is left of my family in Michigan lives somewhere along or near I-69.
There is a RV park in Lansing (Cottonwood) and it is OK if you have a reason to be in Lansing and need a place to stay, but I don't see it as a vacation destination. Somewhat north of Flint, there is a RV park in Frankenmuth, located for those visiting that tourist destination (Bronner's Christmas World). I haven't stayed there, since Frankenmuth is a short drive from a couple of places where I stay with relatives.
Many of the recreational camping destinations in southern Michigan are in the western part of the state, along the Lake Michigan shore or in the dunes east of the the shore, in wine country. The other recreational area is east of I-69, north of I-94, from Jackson to Brighton, then north and east from Brighton toward Waterford in northern Oakland County. It is all around a chain of glacial lakes running from Jackson to Pontiac.
Most of my camping in these areas has been camping, not RVing, in the state and regional recreation areas (Kensington, Island Lake, Brighton SRA, Waterloo, Pinckney). I think if I wanted a classy RV destination, I might go to Camp Dearborn, near Milford in western Oakland County, on the fringe of the Detroit metro area. That park is well run, but because I camped rather than RV'd, I'm not sure about the size of the RV sites.
Most of the people RVing in Michigan will usually be going to someplace north of Clare, if not going to the shores of Lake Michigan or Lake Huron.
Tom Test
Itasca Spirit 29B