Planning Sept trip to Yellowstone/Glacier NP

The trip is fairly obvious, exit YNP via the West Yellowstone entrance follow US 287 north, it merges with I-15 for awhile around Helena. Follow 287 off 15 and continue north. US 287 merges with US 89, follow that to St Mary. One day trip. Mapquest calls it a 7 hour trip.

Fishing Bridge the main YNP RV campground is closed this year so you'll have to use one of the other park CG without services or use a private CG outside the park which will mean an hour or so each way travel time to places in the park.

We have mainly stayed at the KOA in Lake Mary but there are a few other CG and there are some inside the park, again w/o services. At least the KOA is closer than the YNP private CG.

I saw a thread that stated contrary to expectations, September is one of the busiest months for YNP, don't know about GNP. But expect weather at Glacier.

We were at Yellowstone last Sept ourselves, stayed at a Grizzly RV Resort just outside the west entrance. Weather was great, not too many people, etc..(of course there is always the obligatory sightseeing bus full of Japanese tourists, but they don't hang around long) We drove the RV from the west entrance down to Grand Teton NP we have a 31ft class c. We saw plenty of large rigs in the park. This Sept, it's Glacier for us also, we were going to go last year but it was on fire..

We try to make the Sept trip to Yellowstone every year. Weather can be a issue late Sept. 2017 saw daily road closings from snow, with the high country closed for 3 days (Canyon to Tower). Stores etc start closing down also. It's our favorite time in the parks, get some color from trees, animals more active in cooler weather, more relaxed sight seeing. Both times in recent years we went to Glacier toward end of season (Sept), we encountered closed roads from weather and construction. Keep up on the parks road condition web sites and eliminate surprises. Glacier's road to the sun is still closed today from snow, not estimated to be open before 6/22.

The elk rut starts around Sept 19. The best time to be there. Sure it might snow any night. The weather slows down the visitation. Glacier is much better after Labor Day. The routes are no problem. Next time I go to Glacier it will be the east side of the park.

Grizzly RV is a good choice in West Yellowstone. Sure, it's far from a lot of the park, but you'd be farther out if you were to stay in Gardiner or Cody. Grizzly stays open until late September usually, but make sure you know, and have a reservation, as they start closing parts of the CG after mid-September. Expect below-freezing overnight temps by the 3rd week of September, probably earlier as well, and plan accordingly.

As for 7 hours from West Yellowstone to East Glacier, I doubt it, unless you're happy traveling 80 mph with your RV. GPS's usually figure you're going the speed limit. :) If I were driving that route, I'd figure at least 9 hours. (I don't drive over 65 when towing.)

Looks like Johnsons Rv Park at St. Mary on the east side of Glacier NP could work.
I've been saying for a long time I was going to follow 287 from Beaumont TX to Choteau MT. I might do it this year. I'm thinking in the Bigfoot sometime mid August. Too hot here to do anything anyway.