If you're heading north from Redding, CA your route should be along the Oregon coast - Hwy 101. Your 35' will fit in the majority of the awesome state parks and county parks along the whole coastline. We have no problems finding sites with our 40'.
From northern Oregon, stay on 101 through Washington and around the Olympic Peninsula. Cape Disappointment State Park is the first one you'll come to and is great. You could easily tour the Astoria, OR area from the Cape. There's a huge kite festival late summer there.
Just west of Port Angeles, WA is Salt Creek County Park which is a nice base for touring Olympic Nat'l Park. Another favorite area of ours is the Port Townsend, WA area Fort Worden (beach campsites, not the wooded ones) and Fort Flagler have nice big campsites. From there you can take a day tour of Seattle by using the ferry or a whale watching day trip from Port Townsend.
There is just so much that you'll have to move fast for a 3-week trip and back to San Diego. You might want to consider from Redding to get on Hwy 97 up through Oregon and returning to San Diego via Hwy 101. The coast drive from north to south will give you the best views and easy scenic pull-offs. Hwy 97 north will give you a chance at Crater Lake by staying at Collier Memorial State Park right on 97. Then north to Bend and 26 to Portland, OR is a nice drive. You could head directly for Port Townsend area and then head west and south for the coastline touring. Have fun!
