I have camped at Ft. Wilderness the last two Decembers (early December). Last year it was rather chilly, had to wear sweatshirts and pants everyday and had to use my sleeping bag at night. I was also there during the big rain storm that caused some flooding problems at Ft. Wilderness. Fortunately, my site stayed high and dry and my waterproofing worked great! (Fortunately, Ft. Wilderness was empty enough that they were able to move the folks that were flooding.) I actually got to ride Kali because of the rain though. It was pouring that day and I was all decked out in my rainsuit and Tevas, so I didn't care how wet I got!! Rode it 6 times in a row!!
The year before, I was never out of shorts and was able to just use sheets and a blanket on my air mattress. It never rained a drop either,