If you pass through Seattle and need anything please feel free to look us up. We also have a large level yard and you are welcome to overnight. I can also point you to a great city boondock (if that's possible) spot on Alki Beach in Seattle. We have many friend who live on Alki and have overnighted their many time and never been hassled by the cops.
I don't know any great camping spots along the coast but here are a couple of things to break up the drive.
The Tillamook air museum is really worth hitting if you like vintage military aircraft. The place is huge and you will need 4-6 hours to see everything. It is also near the cheese factory which is worth about an hour.
About 20 miles or so South of Tilamook is the Pelican Pub and Brewer which sits right on the beach. It is a neat spot tucked off he 101 and you could probably camp in their parking lot which is also the beach.
Have a great trip.