Specifically for the campsites and cabins, generally yes, they are still decorated into Jan 1. Once you get past the weekend (whenever it is) that follows Jan 1, the campsites tend to empty out of decorations (IMO). By Sunday Jan 4 most of the campers will have left with their Xmas decorations.
However, as
@Teamubr says, the parks are still in their Christmas decorations thru the Disney Marathon Weekend (which has its last race, the full 26.2 mile marathon, on Sunday Jan 11, 2026).
But I did a recent Marathon weekend full marathon (on a Sunday) and let me tell you - we went to MK on Monday (the next day) and ALL the Xmas decorations at MK had been taken down overnight. The Jingle Cruise (from Sunday) was now the Jungle Cruise again.
Bottom line is for your dates listed, you should be good to see FW decorations, campsite/cabin decorations, and theme park decorations.
Bama Ed
PS - the Fort Xmas decorations get taken down some time after New Years (no set schedule) and in not all in one night. But a big chunk is taken down the first night (whenever that is) and any stragglers the next. So if the Fort is decorated with wreathes and lights and such at night, take advantage of it, get pictures, see the sights. Because it could be gone when you wake up the next morning.