Not sure where you are in Washington - but lots of good options everywhere! In the early and later season (when the west-side is cold and rainy), we like to head over the pass to Eastern Washington. Over there, we like Wenatchee Confluence, Potholes, and Lincoln Rock.
On the west side, we stay at Alder Lake (closest to home, so good for short stays), Ike Kinswa, all of the Tacoma Power Parks are very nice, Belfair, American Heritage (private campground near Olympia). Deception Pass State Park is a really cool area to explore - we've ended up staying at a private campground nearby (can't remember the name, but just outside of Anacortes) due to the limited number of sites that fit our size RV in the State Park.
2012 Jay Flight 29QBH
2003 Dodge Ram 2500 Cummins H.O. 48RE
Equalizer 4-pt sway control hitch
Prodigy P2 brake controller
2006 Jayfeather 29Y (past Jayco :))