You could visit the Pacific Northwest or the Northern Rockies but I don't think you should try both. And, as has been mentioned, Crater Lake is a long way from either. Personally, I would not choose to pick up an RV in Seattle. Portland not so bad but Spokane or Boise would be better choices, IMHO. A trip along the Columbia River Gorge via Oregon I-84 would be a good route, then maybe down the Oregon coast. If you make it up to the Olympic Peninsula, a ferry ride through the San Juan Islands (walk-on) would be worthwhile. Beautiful!
It's awfully hard to recommend places to someone else. We all enjoy different things. :) IMO, Glacier beats Yellowstone hands down and isn't quite as crowded.
