I'll throw my thoughts in - it appears you're in Oregon. My very limited experience (one two week period) is that for Oregon State Parks, you HAVE to plan well ahead. I got lucky, and we managed to find a couple of spots with short notice. But many state campgrounds have the majority of their sites on the small side, and the bigger ones are fewer. That means reservations even further ahead, to try and beat all the others with bigger units.
That said, there are a huge number of private campgrounds, especially concentrated around all the popular areas. Yes, they cost more - but the availability of full services and longer sites is far, far better. And many times you can get in with much shorter notice. I did have some nervous times where it seemed like we would have no place to stay, but always found something, so you still have to plan ahead. Just not months and months like you may have to with state parks. Some of our best times in your state this summer were when we stayed at a private campground, but hung out for most of the day in the nearby state park. The best of both worlds!
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