There are three campgrounds located off of SR 12 that put you roughly midway between either mountain....okay you will be closer to Mr. Rainier. Tacoma Public Utilities has two nice parks on Riffe Lake. One at Mossyrock and the other east of Morton (Taidnapam). Don't forgot Ike Kinswa State Park (if you can get in). The Tacoma Public Utilities park on Alder Lake are very nice, but would not be convenient for a visit to Mt. St. Helens. You can make reservations for all of these parks through the WA State Park Reservations site.
