Here is my perspective from our trip this summer, which has been inclusive these parks / areas.
If you go up to 80, you get the opportunity to hit many national parks and monuments. Badlands (2-3 days) Rushmore/Black Hills (3-5 days) Devils Tower (2 nights, so you get a full day there). You need to find out when the Sturgis Bike Week is, and ensure you are not there for it (and the week before and after) just because the entire area is jammed.
From there, you can decide if you want to hit Yellowstone from the north side for a few days (highly recommended).
Decide east (and Canadian) and / or west side of Glacier (we did West, will do East at some point in the not too distant future; note the KOA in west Glacier is very nice, not cheap, but proably the nicest KOA we have ever stayed at and worth the money IMHO)
Glaicer seemed a bit unusual to us, access wise as there is really not much road access within the park. Yes, you have the Going to the Sun road which you need to do on a tour bus (the Red buses for the west are modern versions of the old busses they used to have in the park, and appeared to have very professional and knowegeable drivers; we didn't realize that and ended up with another tour bus company that actually turned out ok), but they only take you up Logan Pass and back down from the west side.
On the west side plan for a day to drive up to Pole Bridge and Bowman Lake. Also some good rafting tours / trips, but again, they are not all the same.
There is also Poulson on Flathead Lake (80 miles below West Glacier) which we'll hit next time too (choice of the big bucks, bring your Prevost Poulson RV resort, or what I also understand is a nice KOA on the same hill).
Missoula is a neat college town if you want a few days in civilization, and they do have a big RV dealer there if you need anything.
Then down to West Yellowstone. Again, Grizzly is a very nice, if not cheap RV park.
Yellowstone is huge, and takes a lot of driving (very slow at times) to see. We did 2 days from the north, and a week at West Yellowstone. We paced ourselves, so maybe 5 or 6 hours at most per day in the park.
At some point, you'll have seen so many Bison and Elk, unless it is a heard or they are right up next to the road, you'll just go "oh well, people get moving, please, it's just one Elk/Bison for Pete's sake, come on, go", in other words, they become just like a cow in Montana......
I'd say 2 months is about right, if you high tail it out and back, and spend good quality time at each desination. Add days, because each of the parks have more to do if you really want to spend time exploring them, past the park and gawk at just the major points of interest.
And spend $80 to get your annual National Parks pass, order it ahead of time so you have it in hand when you hit the road. Note, you'll still have to pay to get into Mt. Rushmore......
You transit stops will be the normal, what we have started to call, "Children of the Corn" Campgrounds...... We don't do Walmarts..........
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