Many of the state park campgrounds are open year round, as are the smaller national forest campgrounds. I just updated our camping map to show the campgrounds that closed in October. This is not perfect (sometimes it is hard to get information about seasonal closures) but most campgrounds have a link to the page: https://maps.northwestportal.com/camp/#8.00/47.408841/-120.273283 I agree that you want to stick to the lowlands and by late fall also probably want to stick to the western side of the state as Eastern Washington is significantly colder and snowier in the winter.