Ft. Stevens is a large park but it's very close to Portland/Vancouver so it does fill up. All of the State parks are very busy on the weekends along the Coast. I wouldn't risk not having reservations. Same goes for Beverly Beach and Honeyman too, big parks but they are all about an hour away from a larger city off of I-5 and they fill up. There are some National Forest CG too, but not all are big enough or reservable.
One of our favorite places is a place called Sea Perch RV Resort, though it doesn't have the "resort" feeling, its not as cheap as an Oregon State park, but if you can get a water front site its worth it. You can pull straight in and watch the waves from your RV or right outside your door.