My favorite spot near Seattle was Tall Chief RV Resort at Fall City WA. Formerly Western Horizon members only park, it is now owned by ELS/Encore and reservations can be made thru Reserve America (for a fee). There are no sewer hookups and satellite tv setup can be a real challenge at most sites because of the surrounding pine forests. However, it is a beautiful setting....and most importantly for me, it is a five minute drive to Costco and Trader Joes in Issaquah and very convenient for day trips to Seattle and the surrounding area.
I haven't stayed there since ownership changed, but some of the online reviews indicate that it is still a good place to stay.
2011 Winnebago Aspect
2005 Honda CRV