New Hampshire to Yellowstone

I'll add the Spam Museum in Austin, MN. I found it fun and interesting even though I don't like Spam.

My suggestions stem from places I've been, but you may or may not be interested in. We are heading to the Black Hills in SD this summer, with another week in the Bighorn Mtns. Not our first trip but always so much to see/do. Here are some ideas:

-- Rock n Roll Hall of Fame Museum in Cleveland (depends on your interest)
-- Wisconsin Dells (the route you mention takes you on outskirts). Touristy for sure, but can be fun to visit shops, golf, mini-golf, restaruants, etc.
-- On I-90 at Worthington, MN, North of there is Walnut Grove, home of the Ingalls family from Little House on the Prairie fame. Small museum and house tour.
-- Mitchel SD, famous corn palace. I've driven by and it's something to look at but never been in.
-- 1880 Town on I-90 in SD. Can't miss it -- signs leading up to it both directions. If you liked 'dances with wolves' starring Kevin Costner, they have many pieces from that film. Besides that, these are all historical buildings from the old west put in a unique town. My kids loved it, and wife and I still stop every time....so much to see in the town.
-- Wall Drug -- laugh if you must, but we love the 5 cent coffee (really), the fresh donuts, and the lunch plate specials. Junk shop for sure....but still fun to roam around and rest.
-- Badlands National Park in Wall SD: worth the drive thru and visit. Something to see
-- Minute Men National Missle Historic Site (near wall on the eastern side): visit a real nuke silo and museum. Something to see!
-- Devil's Tower National Monument: there is a KOA right next door -- something to really see -- hike around the volcano
-- Bighorn National Forest: beautiful. Most campsites have no hookups but there are FU sites in towns on the eastern side of the mtns.
-- Cody, WY: visit the buffalo bill museum of the west. Visit the IRMA hotel, built by buffalo bill. Many other shops and things to see here
-- Old Trail Town just west of Cody: more historical buildings and cabins. Visit Hole in Wall saloon, famous for butch cassidy & sundance kid visiting (not the actors-- the real deal!)

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I would definitely book reservations for this year, especially since so many people have taken up RVing.

I90 is the most direct route but it does take you through major cities. I've done it multiple times. Others may have suggestions to circumvent them but if you stay on 90 make sure you say in 90 express and not 90 local through Chicago. I've done that trip in three weeks (including Yellowstone but not Little Big Horn) so 6 weeks should be fine. Did straight shots out and back so not much advice on what to stop at. While you are in SD I'd go to Keystone. Very touristy and kitschy but fun. Bear Country USA is also very enjoyable. Treat yourself to elk medallions at Jackson Lake Lodge's restaurant in Grand Teton. Enjoy!