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.
Arctic Fox 25Y Travel Trailer
2018 RAM 2500 6.7L 4WD shortbed
Straightline dual cam hitch
400W Solar with Victron controller
Superbumper