Along Oregon US97 there's a decent selection of state parks. Summer Lake is the only commercial place I've used, and that was still pretty rustic (with a old barn like hot spring). By September the parks won't be as heavily reservered. There are also forest service places that aren't too far off the main highway.
The selection isn't quite as good in Washington, though I've used a couple toward the south. And BLM sites along the Yakima River canyon. East of the Columbia it's mostly irrigated farm land, with fewer camping options. And there are campgrounds by the reservoirs along the dammed Columbia.
