We were at Ft. Wilderness for Christmas last year. We made reservations the day of our arrival while we were parked at the welcome station at the Florida border. The loop we were in was not full. There was plenty of room.
Christmas day was crazy at the Magic Kingdom.! I have never seen it so busy and I would not go back on that day. We didn't enter the park, but according to the Disney app, the day after Christmas was dead.
Warning: With the new wrist bands, people with reservations in the resorts can make fast passes up to 2 weeks ahead of time. This changes the entire strategy for seeing the park. Since this was spur of the moment for us, we didn't buy tickets until about 2:00 PM the day before. We immediately tried to get fast passes for the next day. The first fast passes available were 5:30 PM.