September is a very nice time, along with May.
You're retired and have a small RV, why do you feel you need to make a ton of reservations?
We've traveled the Oregon coast in spring, summer and fall without reservations for our 40' motorhome. Of course, we don't require 'resorts' and don't even like RV parks. We stayed in the state parks and even a few national forest campsites on the coast. Some parks don't even accept reservations. If you pull in early morning you can even get cancellations.
Try going without reservations. You'll be surprised how much more relaxing it is. If you want to stay somewhere longer, you can. If you want to leave the next day, you can. You don't have to shuffle around your planned reservations.
If we had to make reservations for every night of our 16 years of constant travel we would have never traveled full-time.
Some parks we've stayed in without reservations are: Beverly Beach, Bullards Beach, Cape Blanco, Carl Washburne, Harris Beach and Fort Stevens.
Full-Timed for 16 Years
.... Back in S&B Again
Traveled 8 yr in a 40' 2004 Newmar Dutch Star Motorhome
& 8 yr in a 33' Travel Supreme 5th Wheel