For Crater Lake both of these are just about equal distance away and both are great with large roomy sites: Collier Memorial State Park on Hwy 97 at Chiloquin, OR and Joseph H. Stewart State Rec Area on Hwy 62.
For Mt. Saint Helen there are two view points and we've done both. One was a private park on 504 off I-5 at Toutle River RV Resort which has large sites. It's a short drive to the Visitor Center.
For a completely different view we stayed off Hwy 12 at Iron Creek national forest campground which was close to Windy Ridge Viewpoint. We enjoyed both. If you stay here it's an easy drive for a day trip to tour Mt. Rainier Nat'l Park from Hwy 12 and Hwy 123.
Full-timed for 16 Years
. . . Back in S&B Again
Traveled in a 2004 Newmar Dutch Star 40' Diesel