The Puget Sound has a ton of great parks, but you didn't specify where (I'm assuming you mean the San Juans) in the area you are staying or how long.
In the South Sound (Seattle to Olympia) you should look at the FamCamp on JB Lewis-McChord, as well as Dash Point State Park. I've stayed at both these sites and they have full hook-ups and can fit most rigs.
If you are looking at Seattle proper, there are several commercial parks, but the only one I can recommend is Lakeside RV Park in south Everett. I work in the area and have driven thru there a few times. It is clean, relatively secure and usually has room for transient traffic.
The North Sound has more options, but they don't always have full hook ups. Cliffside RV Park at NAS Whidbey Island is closed during remodelling, but there are several State Parks (Bay View, Deception Pass) plus plenty of commercial ones (Pioneer Trails is my first recommendation).
I've never stayed on the Islands, so I'd recommend you look at other review sites like rvparkreviews.com and please take a few minutes to write a review of where you do stay. Hope you enjoy your time up here in Sound!
