Walmarts aren't EVERYWHERE. You'll find them, but perhaps not in the most convenient locations for where you'll be going. However, you should be able to find alternative stores, especially for groceries. Asking the locals - at a campground or gas station - where they shop can be quite helpful.
I like the idea of flying into Boise or Spokane and doing a loop from there. Yellowstone & Tetons (next to each other), Glacier, Olympic and Crater Lake are all reachable from there, although you will find you'll wish you had more time, especially in Yellowstone, which is very large & varied (DO allow for at least 3 days there). Not sure I'd describe Boise to Pocatello as "spectacular" but... :) You could take US 20 instead of I-84/86 as an alternate on that route for a little more scenic interest, including Craters of the Moon National Monument, worth a couple hours (it will be HOT in summer).
Before deciding on where to fly to, make sure that it will be possible (or at least won't break the bank) to rent an RV one-way between 2 places. Some companies will not allow that, and all the others will charge an extra fee in most cases.
If you are planning on making this trip this year, make sure you can get campsites in/near the parks you want to visit. Some will be booked up well in advance.
