THe days SHOULD be available, there is no way, if the 7 days you can book right at 11 months is working properly, that the room can ne released into bookable inventory until the last day of the existing reservation. Otherwise they would be overbooking the resort.
You should certainly be guaranteed your room without calling day by day, as long as you call 11 months before your last currently booked day.
For instance, say a resort has 3 GVs, and person A calls and books Nov 1 to 7, person B calls and books Nov 2 to 8 and person 3 calls and books Nov 3 to 9. No GV can be returned to booking inventory until Dec 7, when person A's room is vacated, since that is the start of the 11 month booking window. Therefor, if Person A calls on Dec 6 to extend the reservation, it should be guaranteed, no? Unless the room is taken from inventory for scheduled maintenance, and do they schedule maintenance that far in advance? A lot of their maintenance seems awfully last minute.