@Disneyloverliz,
500 days from yesterday put your furthest arrival date out to December 23, 2023. Assuming they loaded the inventory, it's entirely possible that it is already fully booked. People who are arriving on Dec 22 or Dec 21 or sooner and staying till Christmas (or some even later through New Years) will have filled up the available spots.
As a reminder to others, you can book your ARRIVAL DAY 500 days (or 499 nights but it's the same) in advance up to a length of a 30 day reservation. Disney has recently installed some restrictions around holiday times in Oct/Nov/Dec (that we know of) to making changes to reservations because the method of "book-my-arrival-date-early-like-December-5-till-January-5-then-later-trim-off-days-to-keep-December-23-to-January-1" was being used to excess (among other things).
Even with that, though, the Christmas holiday time at the Fort is the busiest time of the year so I'm not surprised that it booked up by now. Halloween time was the same way. Just keep trying to piece together dates as people cancel at the same level you find through constant checking while keeping an eye on the cancellation window for your rental trailer.
Not a good answer but it sounds like that's where you are.
Bama Ed