We were in Northern Wisconsin, Minnesota and the UP of Michigan in late April this year. Most RV parks were closed, normally opening May 15. The state parks in Michigan were all open, but only had electricity available (all we really needed). Loved the serenity of those mostly empty parks.
One thing we really liked was the newly opened Minuteman Missle Museum. (https://www.google.com/search?q=minuteman+missile+museum&ie=UTF-8&oe=UTF-8&hl=en-us&client=safari) just east of Wall, SD. We stayed at the KOA in Interior, SD (https://www.google.com/search?q=interior+sd+koa&ie=UTF-8&oe=UTF-8&hl=en-us&client=safari) and it moved to close to the top of our favorite's list.
Have a great trip. Extended trips are certainly the way to visit this great country, see some epic things, and meet awesome people.
Be safe,