Hi Connie,
Christmas time in Walt Disney World is certainly magical! I have no doubt that you and your family will make some wonderful memories on your first Disney vacation.
I'm not sure if you've looked into this or not, but
Fort Wilderness Campground would be a great place for you to stay. When you call to reserve a campsite, simply specify the RV's length and any other details about its size (slideouts, awnings, etc.) so that you can be assigned the right size campsite. You can either book online or call (407) 939-6244 to speak with a reservation specialist.
There are tons of activities at Fort Wilderness: swimming, horseback riding (for a fee), bike rentals, tennis, playgrounds and more. There is a nightly campfire sing-a-long/movie night where Chip and Dale make an appearance.
Holiday campers even have their own, unofficial, site decorating contest. Some of the regular campers get quite elaborate with their decorations, and it's really fun to see. The Resort also has horse-drawn "sleigh rides" that takes you through the forest and around different areas of the resort.
Hope this helps. Have a magical vacation!