The latest I would leave for seeing the northern central states is mid August. Then take 4-6 weeks to meander around up there because the weather can get real cold real fast. The end of September can be unpredictable. You're in Michigan so you know about driving in snow...but towing in snow? I used to live in Michigan also. I don't miss it. Mountain passes can be closed in advance of snow coming. You will find out that 6 weeks is not enough time to see everything too.
Download the RVPARKY app. We use it when on the road and the app gives you real time location and RV parks nearby or on the route. We look for a park far enough ahead to get off the road by 4pm. We start calling parks about 2pm when we find one we think will do. Never been without a place to park it. Plan on staying 2'to 3 nights at one park every few days...or when you get to an area you want to explore.
I also have gone online and ordered the tourism info for all the states we are going through. Yes, It's a lot of paper but I find it better to have planning material in front of me rather than just looking at it on the 'puter.
We did the 'big circle' trip a few years back and LOVED it and want to do another one. 9,400 miles in 10 weeks. We did not see all we wanted to. Gotta do it again now. Aw shucks.