Confirming what ed said... I just spent time looking at "Weeks" of 5 days (Monday - Friday) and I was able to find various openings other than the Tent/Pop-up sites... If I added a weekend, it was a No-Go. In come cases ALL of them were blocked, including the Tent/Pop-up.
Again, the best bet may be to call and try and "piece" it together. Of course that's brutal when trying to make (and more importantly CONFIRM) your plans.
Often times as the date approaches and other peoples plans change, they cancel and then you can get the dates you want pieced in. (of course you need to check REGULARLY, online and calling) Down side is that you may have "Multiple" reservations (meaning multiple deposits), and if the reservations are not the same type, you may have to move between them. This is the challenge of having a longer reservation. Any other place you'd think 6 months in advance would be good, but not at the fort.
Hope all works out well for you.