Say you're coming out of the Chicago area.
About 2 hours out puts you in South Haven.
Sunnybrook is a good rv park just east of town. A number of things to see and do in the area including beaches, blueberries, SH shops, Saugatuck shops, Grand Rapids, etc.
I-94 between Chicago and Detroit is my least favorite road. Numerous big trucks and commuters who think 80 mph is slow. Its also a rough road. If coming from the South and don't have to do Chicago, then I like US-31 out of Indianapolis. Along that route just SW of South Bend, IN you will find Potato Creek State Park. I can't speak for Indiana, but Michigan State Parks are difficult to get into. Plan well in advance for those.