Washington state is a bit complex when it comes to camping. You have fed, state, local and private camping. I am yet to find a single resource that is accurate. I have used maps, guide books and tapped local resources.
The good news is that there are many great sites. Many areas have multiple campgrounds with in a ten mile radius. There are a few questions that you have to answer if you want to have a good time:
1. What do you want to bring? (We offen bring our own water and wood.)
2. What mobility issues does your party have? (Our case cliffs bad with toddler).
3. How many people do you want to see? (We perfer fewer so we stick to the Eastside).
4. Do you want the facilities to have anything special. (i.e. do you want an electrical hook up?)
I tend to find a site on a map or guidebook and then check it out prior to camping there. This can be a simple checking out the area from a known good site to a quick day trip. One of the guidebooks that I use can be found here:
http://raysguides.com/index.htm . I hope this helps a little..