In the Seattle area there really isn't much for RV parks. There is one East of Seattle very near the freeway that I looked at several years ago and rejected. We were traveling with two indoor cats, and they had a one pet rule. I don't remember the name of the place.
We wound up across the bay, near Poulsbo. Very nice little RV park, with back-in sites around the perimeter for long term stays. DW had to ride the Bainbridge Ferry to get to work and back, but it worked out quite well! The half-hour nap was welcome. Eagle Nest RV park, or something like that. It is in the Woodalls Directory.
There was an RV park just South of Everett that would have been OK, but they also had a rule I did not like, concerning what I was allowed to have in my RV. None of their business, IMO.