Not sure how big your RV is, but the Kent KOA near Seattle has very few big rig sites and all of them are super close together and it is very hard to maneuver the turns inside the park. In Seattle area, there are not many parks to choose from and all have sites very close together.
We stay at Issaquah Village RV Park in Issaquah, WA, just 15 miles east of Seattle. It is the nicest park in the area and is a short distance to Seattle so that you can do the touristy stuff. Some spots are larger than others. Spring through Fall books out pretty far in advance. No matter which park you choose, the parks stay full and you will have to have reservations. Just this year, over 1,000 people were turned away each week from the Issaquah Village RV Park because there aren't enough RV parks in the area to meet the demand. This will be even worse in 2016 because Snoqualmie River RV Park has been sold and will not be reopening.
If you know your plans, make reservations now or no later than January for the Seattle area.