Best times for Michigan, Minnesota & Wisconsin?

Depending on where you are going in Michigan, August and September are pretty good. West Michigan doesn't get too many 85+ days--especially along Lake Michigan and the UP is almost always cooler. September in the UP can get down right chilly. Hope this helps.

http://www.usclimatedata.com/climate/michigan/united-states/3192

Depends in Minnesota on where. Further north you go, the colder (duh!)
For the far north (near Canada) very late May and all of June, or September to early-October are the prime periods. I've had some cold (but not freezing) fishing trips up near International Falls the first week of June, but mostly 60-mid 70 degree days, with 40-50 degree nights. We've had very rare blizzards as early as Halloween in the Twin Cities, but usually late into October is great mid-state and south. This past two weeks near Bemidji (mid-north) it was great - lows around 50, highs in the mid-70's to 80. July is usually good all over, although getting warmer further south. August can get steamy - high humidity, temps 80-90 plus. Cooler up north.

Wisconsin would generally follow the same pattern. Close to the Great Lakes, things tend to moderate - not so hot, but not so cool, either.

The one year we went to Maine (Acadia) in May, it was about the same.

The UP and northern half of WI are great to visit anytime. My personal favorite times of year are late May/early June and October.

In the UP, visit Munising and the Pictured Rocks National Lake Shore, and the Porcupine Mountains area. On the west side of Delta county is Fayette State Park which is essentially an abandoned mining town. Very cool place to see. It rarely get hot in the UP. 90 is an extreme there even in July/Aug.

In WI you'll want to spend at least a few days in Door County, the "thumb" of WI. Unless you have kids, I'd avoid the WI Dells. Very cheesy and commercialized (think smaller version of Pigeon Forge, TN). Fall is fantastic in the Door. Great scenery most every where you go. Don't miss Peninsula State Park and Cave Point County Park. Stay in Egg Harbor or Fish Creek and most things are within a 20 minute drive.

Thanks for the suggestions! I'm making a list :-)

The UP in late December is beautiful, especially as Lake Superior starts to ice up. Around Munising there's several waterfall within reasonable drive and IMHO they look more beautiful frozen than they do flowing. Snowmobiles rule the UP in the winter, more of them on the roads than cars, if you have some bring them (or rent them when you arrive)

I's divide the trip into two separate trips. There is much to see and do in the Midwest ditto the NE. One could easily spend a week in each midwest state. We spend a week in Door County, WI every year!

Both regions have similar climates so your window is narrow. Why not do 1 region 1 year (and South for the colder months)? Then try the other region the following year?

(You want to be out of Minnesota by Oct 1)

We were hoping we could work it in one year since we will be starting from Washington State, but it does look like we will need to break it in to two trips. It looks link fall is a bit warmer. How would you head south, and how soon?

I'm going to tell you the way I see it. The UP in mid September to early October is stunningly beautiful. The crowds are sparse. The weather will be very cool at night; could get down to 30°±. The days will be very pleasant 60°s to mid 70°s. In my opinion, your "no freezing weather" means you want to miss out on the absolute best time to visit the UP. Your RV has a furnace, being from Washington I presume you have a variety of jackets. It's really a no brainer, it's not like it's going to be 40° below.