Yea, we've been to all those parks in September and found that availability is really hit-or-miss. You are undoubtedly aware that the Going-to-the-Sun Highway in Glacier NP closes for the season on Sept. 21. Tea at the Prince Edward Hotel at Waterton Lakes NP is also out after mid-Sept. as the hotel closes for the season at that time. We didn't realize that the last time we were there and regretted not being able to go inside. The Yellowstone NP campgrounds begin shutting down on Sept. 1 and all but the Madison are closed by Sept. 21. And even the Old Faithful Inn closes on Oct. 5, but is a great place for lunch if you get there in September. But no matter the time of year, don't miss the Buffalo Bill Historical Center in Cody--it is well worth a couple of nights in a private RV park.