I will let the others chime in on the two areas you mentioned, however, if you are going to Mt St Helens area, "Brookhollow RV park" is a nice place and within a few miles of the St Helens visitor center. The park is locatedone mile fromexit 39 on I-5. That is where we stay when visiting the wifes parents place.
Other places to visit may include Leavenworth (just west of Wenatchee on hiway 2 I believe) or Deception Pass on Whidbey Island (I am biased cuz I live on the Island!), There's also a lot to do in Seattle but not sure where the campgrounds are. Enjoy your trip.