Oregon has some nice state parks. We were able to reserve FHU sites at Champoeg SP (between Portland and Dallas) and Tumalo SP in the outskirts of Bend, both wonderful parks. Other sites with electric & water or no hookups available too. Tumalo isn't as good with big rigs as Champoeg. Champoeg (pronounced "Shampooey," seriously) is a great location in the Willamette Valley, not far from McMinnville (the Evergreen Museum is a must-see) and not bad for a day trip into Portland, plus just some nice country drives in the area.
We drove across the state on I-84 and spent a couple nights at Boardman Marina RV Park (county-run, as I recall?) Great views, worth reserving a back-in site along the Columbia River if you can.
As for visiting in the last half of September - perfect!! That's when we were there and the fall color was just starting, weather still pleasant - we loved it.
2020 Toyota Tundra CrewMax 5.7L V8 w/ tow pkg, Equal-i-zer
2020 Lance 2375