We usually start planning for next year the day after labor day and start making reservations from then until Jan or Feb. This usually puts us ahead enough in most cases to be able to get in (our small size helps out there too).
Exceptions being popular places at popular times (e.g. holidays, WDW Ft. Wilderness over the Christmas break, etc.).
If you were just trying to "wing it on the fly" calling around at the last minute you may find a cancellation or two that can let you sneak in.
A few others that may get filled as being popular:
Normandy Farms, Foxboro, MA
Mt. Rushmore KOA and Palmer Gulch Resort
A good resource is RV Park Reviews
many of the reviewers leave comments about how difficult it is to reserve there.
(Note that RV Park Reviews is run by the same people that run the Thor Forums here.)