This may help you, go to Campground Search and type in (55+ park). Do the search and this will pull all the 55+ parks we have listed. I found 136 hits.
If you're looking for something in the Seattle area, I'd recommend Issaquah RV Park in Issaquah. It's not a 55+ park but it doesn't seem to attract families (there's no playground, pool, rec hall, etc). There's not much to choose from close to Seattle and I think this is the best. We were there a month last summer and it was convenient to everything. They fill up fast though, so reservations are essential.
Thanks to both of you for your responses. Any others are certainly welcome as well. I just did a search and there are really slim pickings in the NW. My guess is the reason for that is most seniors seem to be interested in a place they can full time while people like me who move around a little don't care as much. I guess I'll just have to wait to see if there are any spcific recommendations and just keep on searching myself.
Astoria, Oregon has a nice, smaller RV Park that caters to adults: Lewis and Clark Golf & RV Park. Not specifically 55+, but no bath houses, pool, or planned activities for children. (Astoria has multiple other resources for folks with kids, including a monster State Park.) We stayed at Lewis and Clark Golf & RV Park a couple summers ago. The owners are top notch. They both enjoy giving their customers a great list of things to do and see in Astoria. What started as a week, turning into the best summer trip ever. Great tow-car side trips! Cheers!