We drove from Seattle to Glacier in one day. Last minute road trip in early August. We did no planning other than what we wanted to do in Glacier National Park.
Stopped at Kalispell for groceries and White Fish for dinner. Decided that was as far as we were going to get for the night. Made some phone calls for a tent opening and no one could accommodate us but Sundance. It's pretty packed together, bordered by a fence on the side. Tents on the outside, trailers on the inside of the U shaped loop.
The good: The folks in the office were super nice. Told up we could pick anything that was free at this point. There is power and water at the sites, some have a shelter built for hammocks or tents. The bathroom had showers and stalls like a KOA or Encore campground. It was pretty nice!
The not so awesome: Our site was down on the far end. Close by train noise every 40 minutes or so. Our site was cramped and lot very even. Our table was so warped it was unusable. The bathrooms were a bit of a trek uphill. The trash was overflowing and pretty gross, we picked some of it up.
We were only there to sleep. Got in, set up, went to sleep, packed up at 5am and headed out to Glacier. It would be a fine base for GNP activities, maybe best in a trailer were you weren't relying on provided picnic tables and could drown out the train.