I really think you'd be able to find something at that time. However, plan to come into the park a few days prior to the beginning of the holiday weekend. As you get closer to the time you might feel more confident calling for a reservation. With your size RV a good bet might be Fort Stevens State Park which has a lot of sites, the Carl G. Washbourne State Park at Florence, OR or further south at Cape Blanco. Those three areas aren't as heavily used as some of the other areas. Also around Florence are some nice forest service campgrounds. There are a few casinos along the way and lots of private RV parks. You've got some research to do. Good luck!