No one has mentioned Gig Harbor RV resort. It is in a good location for dropping a big rig and touring the Seattle area. For the coast, take a look at Ocean City RV park for being near Ocean Shores, although the Grayland beach is better. The Quileute RV park in La Push has a few long pull throughs. It is good for seeing the Hoh & has beautiful beaches nearby. I would suggest driving to the Southworth ferry in Port Orchard to Seattle. You can walk on and park the truck at Southworth. You could also take the ferry from Port Townsend if you were heading to the north Cascades. Different choices and different routes. Enjoy!