If you are going to be staying in the area of Seattle, you will have to have reservations. I work at Issaquah Village RV Park a couple of days a week and we turn away from 500 to 1,000 people a week from April to September because there aren't enough parks in the area for the demand. It will be worse in 2016 because one park closed and the property has been sold. Another park just recently sold but we don't know when they are actually going out of business.
We are only 15 minutes east of Seattle and a quick drive to Seattle in order to do the touristy stuff. So, please, make your reservations now!