I've been to Montana several times (have friends there). Glacier is about the only must see, as far as I've experienced. But I'll add a few other things to see/do.
Lewis & Clark Caverns State Park (with campground) in Whitehall
In Bozeman, Museum of the Rockies and a really neat little Computer Museum (you don't have to be a computer nerd to enjoy).
Great Falls has a very good Lewis & Clark Interpretive Center.
In Fort Benton, also a very interesting Museum Complex.
Can't help with campsites...we stay on our friends' property and drive the car around.