We have gone back to Oregon a lot over the years, I grew up in the Gorge and my wife is from Portland. There are a lot of RV parks both public and private in the Gorge. Most of the State parks have some sites they don't reserve if you get there mid afternoon you have a good chance of getting a site. There is also a county park near Hood river and plenty of private parks on both sides of the river. A few calls should locate something. They all have either train or highway noise or both. Over coast is a little harder and very busy in season but we have done it without reservations, in season, lots of options just don't wait to late in the day.