gal/claire,
The Fort gets more full on President's Day weekend (and most weekends are more full due to weekend guests coming in). The weather in February is pretty nice - warm in the sun, cool in the shade, not much rain that time of year.
If you were aiming for a weekday night (Sun-Thur) I would say you could walk up. Weekends probably need a reservation (but you can make it closer to time). Cancellations can be made up until 5 days before arrival with no penalty.
There are two primary Tent loops (1500 and 2000). Ask for 1500 - its general location is closer in.
Rates go up after the first week of February so the rate will vary across the month depending when you go. There is also an upcharge for weekend nights (about $5-6) versus weekday nights.
Bama Ed