If towing a TT, stay clear of the Seattle area from around Everett to south of Olympia, including the 405 around the east side of Seattle. Traffic is a total zoo and is tiring, nerve racking, slow and can be hard on a truck & TT. I think CGs close to the Seattle area can be hard to get into because there's a lot of workers living in RVs in them. Check on websites like rvparkreviews.com for possible info.
We're Thousand Trails members. They have a wonderful CG in Chehalis, about 1/2 hour south of Olympia and could get you to/from Seattle for a day trip. Chehalis is within striking distance of the south access to Mt. St. Helens. Have been to a CG in Randle, which is close to Mt. Ranier - very pretty area and away from the madding city crowds.
Gotta drive up the 101 on the east side of the Olympic peninsula. Great scenery and drive, sort of like the Oregon Coast in a way. Lots of CGs there. There's a waterfront CG at Port Townsend, Ft. Worden State park, an old military fort - great location. Also gotta drive up to Hurricane Ridge visitors center out of Port Angeles (hopefully on a clear day). Many excellent CGs along the WA west coast.
Last fall I towed our 29' TT myself from Everett to south of Olympia east of the 405 while en route to Chehalis. Can't remember the exact route - road was quieter, twisty in some spots, scenic and took me through some lovely small towns. Great drive and would do it again anytime, but is a slow route.
Depending on the time of year, wildfires can be a problem in WA (and CA, OR & BC) from about July to Sept. It's possible routes may have to be changed along the way and some CGs may not be accessible. There's been some pretty bad fires in recent years.
