1) Cancellations do happen at The Fort.
2) But, are far fewer than other resorts.
3) Campers tend to plan more carefully, and seldom change their plans.
. . . most take great pains to schedule when other things will not interfere with camping
. . . last minute money issues (typically) do not get in the way, as most RVer's are somewhat well-healed
. . . camping, to many, seems to be more fun than "just a WDW vacation" and folks are loath to miss it
4) So, if waiting for an opening,
. . . check 2-3 times per day and hope someone has to change plans.
. . . try to avoid night near holiday periods (eg. Halloween, Thanksgiving, Christmas/New Years, President's Day)