You didn't say where you were leaving from (you might want to add that to your profile) but some nice places we've stayed at on that route are:
Flying Goose campground in Fairmont, MN
KOA - Mitchell in Mitchell, SD (close to Cabelas)
Sleepy Hollow Campground in Wall, SD (walking distance to Wall Drug)
Bighorn Mountains KOA in Sheridan, WY
Bozeman KOA in Bozeman, MT
Fidalgo Bay Resort in Anacortes, WA (Excellent location and an easy day trip to Seattle. Plus it's a great jumping off spot for the San Juan Islands, Whale watching (Orcas) tours, etc).
I'd recommend NOT staying in Spokane. There really aren't any nice places there. I'd stay in Cour d' Aliene, ID if Spokane is about where you'll end up some evening.
If you are going south to Mount St. Helens, the Columbia River RV Resort in Woodland, WA is excellent.
If you want to cross down into northern Oregon, the RV Resort at Cannon Beach is in Cannon Beach, OR and is a first class park and Cannon Beach is a very nice place to see.
Last time we got off of I-90 at Spokane and headed up through Grand Coulee Dam and then went through the North Cascades and headed west towards Anacortes. It's a beautiful drive and the picture below was taken near Washington Pass.
We also stayed at Fort Worden in Port Townsend, WA. Fort Worden is an old coastal defense artillery fort from WWII era that is now a state park. You can camp right next to the beach there.
All of these places have websites. If you can't find them via Google, feel free to send me a private message and I'll get you links to them and answer any more questions you may have.
Mark & Leann Quasius
2016 Cornerstone 45A
2007 Allegro Bus 42QRP (Sold)
2012 Jeep Wrangler Unlimited - Rubicon