Devils Tower >>>>Gardiner Mt

Actually, there are several places to stop right off of I-90. Starting with The loop road through Badlands N.P. When it comes back to I-90, you're in Wall, SD. home of Wall Drug Store. You will have been seeing signs for it about the time you pull out of your driveway. They advertise free ice water on the signs. The place is huge but you have to stop if for no other reason to get your picture taken on the concrete statue of a Jackalope. A jackalope's bottom half is a jack rabbit and the top half is an antelope head, complete with horns. Rumor has it that they're only seen late at night about the time the bars close.
Next, is Rapid City, SD. Head south on 16 to visit Mt. Rushmore, Crazy Horse Monument, Custer, SD, and Wind Cave.
Shortly after that, you'll see the detour for Devil's Tower. A huge rock with the top chopped off. That's where "Close Encounters of the Third Kind" was filmed. Next stop on I-90 will be the Little Bighorn Battlefield where General Custer severely underestimated the size of the enemy.
Then continue on I-90 to south on US 89 to Gardiner, MT. That's the north entrance to Yellowstone N.P.

There is a very nice RV site right inside Badlands NP. And a nice KOA right at the base of Devil's Tower. And from little Big Horn I stayed in the original KOA in Billings, then on to Gardiner the next day.

Good info about the Badlands & Wall Drug, but those are east of Devils Tower by quite a bit. :) Between Devils Tower & Gardiner, the best stop along I-90 (IMO) is for the Little Bighorn National Battlefield. A couple CGs rated well are nearby; we stayed at the Hardin KOA, which was pleasant & a mile off the interstate (= less road noise). It's a good day's drive from the north entrance of Yellowstone.

The KOA at the base of Devil's Tower has a view that won't quit. Worth the stay.

kknowlton,
Since he's starting from Wisconsin, he'll be headed west.