If you can't get into a campground, there are some well-located RV parks near the mouth of the Klamath River -- easy drive to Prairie Creek and the other redwood parks in the area. We liked Kamp Klamath -- not everyone does. It is kind of funky and run-down and "homelike." (I guess that tells you something about my home, doesn't it?) They have a barbecue on the weekends with really good live music. The sites are large and grassy. No traffic noise, which is nice. Try to get a site under the trees, rather than on the big open lot -- talk to the park managers before you book a site. Check out the Klamath entries on rvparkreviews.com -- there are several good choices.
From the bluff near the park, you can usually see whales feeding along the beach -- this is a "resident" pod that does not migrate from Alaska to Mexico -- very unusual.