don't see why you couldn't. you can try calling them at 407-WDW-CAMP if you want to be sure(I would recommend). But you can pay for everything else with your rewards so I would assume so.
From personal, first hand experience, YES, you can. Value seasons for the Fort are generally the first few months of the year and Fall, but you can check the disneyworld website for exact dates.
Great news! Thanks! As of now I have $310. in rewards. We plan to go to Florida for several weeks this winter, in our motor home. I thought it would be a nice surprise for my DH to stay there for a few nights and want to get the most for my $$$.