We are just back from a 3-night stay in Glacier NP, at the Fish Creek CG. We were assigned the B-loop with an advance reservation. The B-loop sites are nice and woodsy but many are narrow pull-throughs and/or not level. Most of the sites in the A-loop are level, plus this is the loop with the showers! C-loop doesn't take generators. The D-loop looked OK site-wise, with the advantage that some sites overlook the water (L. McDonald.)
Advance revs. are highly recommended for July and August, which are available for the Fish Creek (West) and St. Mary (East) CGs. All but the "primitive" NPS CGs were full every night. We noticed that the Glacier Campground RV park in West Glacier had space for tents-only on the day we drove in.
The "primitive" CGs are along places like the Pole Bridge road along the west side of the park. It hasn't been graded for some years, so would be fine as a day's outing with a high-clearance tow-vehicle, but we sure wouldn't haul an AS up it, although the CGs at Kintla and Bowman Lakes have gorgeous scenery. (We are going back to Kintla Lake minus the Bambi at the end of the month for a combined canoe and hiking trip.)
Most of the campgrounds inside the park this year are closing on either Sept. 2 or 8th. Probably due to govt. budget cuts.
Going-to-the-Sun road was jam-packed with traffic, with no parking available by mid-day at the summit (Logan Pass) visitor center. There were also long lines by mid-morning of people coming into the park at the West Glacier entrance station. Would highly recommend doing Going-to-the-Sun either early in the morning, or else taking one of the Park Service shuttle buses or concession touring cars later in the day, with their frequent pick-up and drop-off schedules and minus the parking hassles.
July or August seem like the best times to be there (due to snow lingering into mid/late June) but this does compress a lot of tourism into a very narrow time period.
We love Glacier NP!