From entering at Sault Ste. Marie you could head south to St. Ignace or Mackinaw City and take the ferry for a day trip to Mackinac Island - IF it's a nice day. No vechicles are allowed on the island but it's easy to get around. They also rent bikes. I assume you don't want to get too far south in Michigan so you can turn around and head north again.
Then take Hwy 123 up to Paradise and you could stay at Tahquamenon Falls State Park. Regardless, continue past the park to Whitefish Point and take in the Great Lakes Shipwreck Museum which is very informative.
Then head to Hwy 28 to Munising. From there you can take the boat trip to Pictured Rocks Nat'l Lakeshore.
Continue on 41 from Marquette and go up to Copper Harbor. Fort Wilkins State Park is very nice. Turn around and come back on Hwy 26 and you can do Porcupine Mountains State Park. The west end of the Upper Peninsula is waterfall area. Before you get too far west, stop at a Visitor Center and ask for the waterfall map. Perhaps you can work in some of the falls and many are just a short walk. They're not huge all falls are pretty. We're from Michigan so that was our reason for taking Hwy 2 across the states.
You won't be in Wisconsin long but if you like cities, Duluth, Minnesota is nice to explore.
Perhaps someone else can come and give you some Hwy 2 things to stop in Minnesota and North Dakota. Since you're exiting at Northgate, ND I can't help much more. In my opinion North Dakota in that area doesn't offer a whole lot.
Now if you have the time I'd definitely recommend going through Montana, Idaho and into Washington. You'd find lots more to do in gorgeous scenery.
Have I got a plan for you!!! How about leaving on the trip in June and go over to the Pacific, come down the Oregon coast and California by Fall and spend the winter somewhere in the warmth, such as Arizona (80 degrees F today) or over to the southern states. Then by April make your way back home again.
For the stops along the way definitely check out http://www.rvparkreviews.com/
for campground ideas. We happen to enjoy public parks the best but there are plenty of RV parks along the way.
Here's some good information on Hwy 2:
Have fun planning!