Our Disney Family has been going to the Fort for over twenty years for all holidays except Halloween and always in the cabins.
The reason you don't see many decorated is for the previous reason, guests aren't there long enough.
But the cabins are perfect to do the simplest of Christmas Wreaths and Bows to the more elaborate of blow up animated characters.
Treat the cabin like your own home, the few rules Disney has is keep decorations on your site, do not interfere with other sites or travel ways. No damage to the building or decks ie; nails. Finally make sure that your display does not interfere with others guests relaxation, No intrusive lighting into other cabins or music and other noises late.
Have fun and do not forget to tour the campgrounds, we stay in the cabins but by far the best decoration display is in the camping and RV sections. Also don't forget to look up the guest can get imaginative with lights in the trees.