I did quite a bit of Oregon without reservations last summer. A couple things worked to my advantage-small RV fit into leftover spots and stopped fairly early in afternoon-3PM plus or minus. At times at lunch I'd call a private park and get reservations for that evening. Early stop also works well for forest service campsites where there can be level and grossly unlevel sites in campground. More choices. At least for me, setting less ambitious goals for daily mileage allows earlier stopping time and easier travel if you have no reservations. I'm more likely to reserve a spot if I'm pushing the travel into late in a day. In a worst case scenario you can usually find RV parks a little less desirable but decent enough which have vacancies.
