The Boeing plant you refer to is in Everett, which is north of Seattle, so I thought you might be interested in possibilities in that area.
There's a really nice County park near there that, though it doesn't have a swimming pool, does have a beautiful swimming beach/area at the warmwater lake it's situated on. Rates are around thirty dollars a night for hookup sites- no sewer hookups, but there's a dumpstation at the nearby Monroe Fairgrounds, which you'd have to pass by on the way out anyway.
The park's name is "Flowing Lake County Park" and pictures/information can be found at
Snocoparks.org