If you can't get reservations at any of the mentioned rv parks, there is Leisure Acres in Cleveland ( about 7-8 miles south of Helen). We stayed there in October a couple of years ago--FHU. They have a website.
We went specifically for Oktoberfest--had a meal and enjoyed the entertainment in the beer hall in Helen. I loved roaming around downtown Helen...the buildings and stores are so unique.
October is such a beautiful time to go--don't miss the waterfall and the little covered bridge in the area. Helen has a really nice visitor center and they can give you directions.
Also, if you happen to be a Nascar fan, you will probably enjoy the Dawsonville Racing Hall of Fame.