Three weeks is a pretty good timeframe for that itinerary. I'd suggest going via I-90 one direction, visiting the Badlands & Black Hills in the process. Devil's Tower isn't terribly far off I-90 in eastern Wyoming, and there's a decent enough KOA right by the tower, with a great view from many campsites. (1-2 nights there should be sufficient; 1 would be enough if you're going there right from the Black Hills.) Another good stop along I-90 in southern Montana is the Bighorn Battlefield National Monument (Custer's Last Stand). Worth a day, IMO.
You MUST have reservations in/near Yellowstone. CGs inside the park may be booked up by now, and only one (Fishing Bridge) has hookups. Decent CGs in Gardiner, MT (north entrance) and West Yellowstone (west entrance) may still have openings, but I wouldn't delay. I would suspect you will need reservations near Glacier as well. Remember that Glacier has a short season, compared to Yellowstone (and certainly compared to southern Indiana!) When were you planning to visit?
Along I-80, which would be a good way to/from SLC, there are a lot of pioneer-related sites, including "living history" villages, state parks, etc., in central Nebraska, which are really interesting if you are into history. Not especially cheap or simple to get to, but the big arch museum over I-80 in Kearney, Nebraska is worth the stop.
I second the must-see on the Tetons. Allow yourselves at least 4 full days in Yellowstone & the Tetons combined. (Tetons make a good day trip from Yellowstone; the parks are virtually contiguous.)
Between Yellowstone & SLC, the easiest route is via I-15, but a more scenic (and longer) route goes through Star Valley, Wyoming, by Bear Lake and through Logan Canyon. Worth it if you have the time. There's a nice little museum dedicated to the Oregon Trail in Montpelier, Idaho, along that route, and is worth a couple hours.
