I agree with the response so far and think four weeks will be to short especially for being retired
The west has one thing, BIG and a long distances with many potential stops along any of the routes suggested. We did this trip in the opposite direction and Hwy 2 was fun with some off the beaten path stops like Fort Assiniboine. We were warned not to travel trough Williston ND many times when fueling so we diverted south but I suspect the frenzy is over and it may be a bit easier now.
We loved Grasslands NP and intended to stop for one night and stayed several days but at that time no water or dump available just electric.
Loved the I80 truck museum, well worth the stop, Jim's probably been there.
My take on Yellowstone is to stay at West Yellowstone KOA and if you are traveling the 80 or 25 enter through Grand Teton NP and if you are on the 90 enter through Gardener. I exited Yellowstone from the east entrance and we traveled some of the steepest grades we have ever encountered. Slow go from that direction.
If you visit Grand Teton be sure to do the raft trip on the Yellowstone River. We also did the raft trip out of Gardener but not as exciting.
Glacier is nice but make sure you get a nice park, some of them are grim. Traveling late in the year should make this no problem but winter comes early to this part of the world.
On the west coast visit Long Beach Washington and if you have 4WD drive the beach, longest in the world says the signs. Astoria is great for a long day visit, Goonies house is now discouraging visitors so I guess that is off the list but all the Lewis and Clark sites are great.
Oregon coast is the best, if you can stay in Netarts you will find good clamming. Many lighthouses to visit. Fort Stevens at the mouth of the Columbia is a great place to visit. Staying in the park is less expensive and full hookups are available but Mosquitos can be bad. KOA across the road has fewer mosquitos.
Be sure to stay a night or two at Pacific Shores in Newport Oregon, pricey but nice. We also love Seal Rocks RV park. Right across from the beach and the tide pools are great. Boiler bay should be churring by late Sept so you may see the ocean roaring.
Crab fishing in Newport is fun by the cannery.
Traveling down Hwy 1 is great and everyone should have the opportunity at least once. The remaining Redwoods are a site to behold. Lots of wineries along the route.
The admonition to carry bear spray is important even for short walks in Yellowstone or Glacier. Canada may confiscate your USA bear spray and then you get to buy another in Canada? Also wise on Oregon or Washington trails.
Seattle and Portland are both great cities as is Victoria BC. Having a cocktail on the veranda of the Empress Hotel watching the Sunset is recommended. Butchart Gardens is also an experience in grandeur.
Powell's book store in portland is an experience. The Rose Gardens in Portland are beautiful and you will want to visit the Grotto, National Sanctuary of Our Sorrowful Mother. I am sure I can think of more places but you get the picture that the history of America is boundless in the west and Northwest
Custer Battlefield has tours by the rangers and by the local tribes. Did both tours and loved the narrative from both views and how history is finally coming to terms with a shameful period in our growth as a nation.
Dinners ready so no more for now