myjourney,
If you call the FW front desk that far in advance, all they will likely do is go into your reservation and add a note. What I've done every time is to add a request to the reservation when I am talking to the WDW Cast Member when making my reservation over the phone. That request will download to the Fort along with your data when it is time to pre-assign campsites.
I used to fax but the request on the reservation has always been seen and granted (or at least acknowledged and I was told, "I see you requested xxx which we couldn't do but we did do yyy"). So no need to fax - call the WDW reservation line and ask them to add a request.
Finally, when you are at the Fort, you've checked in and you're all excited driving to your spot, you might get to it and be less than impressed (it's happened to me). What you can do is drive around and find an empty site you like (of the same type), note its number, then stop at the Comfort Station and dial the FW front desk from the house phone by the laundry room door(number on a sign next to it) and say, "will you please swap me from my assigned site to site # xxx?". Or call the number directly from your cell phone. My experience has been that they will swap you and there's no need to go back to the Check-in to get new cards, etc.
It will work out!
Bama Ed