I get the feeling the Fort has been burned in the past by people who parked first and DID NOT stay second. Your request is very reasonable but sadly, however, it doesn't always happen that way.
1) Yes, we have been burned.
. . . when asked for such parking, we usually say "No" if you want to park BEFORE another resort/cruise stay
. . . if there is room after their Fort stay, we do allow some to leave the RV in the Main (aka, Oveflow) Lot.
. . . but, during busy periods, there is seldom enough room for such parking
. . . so, do not plan upon this as a alternative
2) People would use their RV/trailer to travel to WDW, but not use it to stay at WDW.
3) They would park it at The Fort, bascially for free.
4) In some cases, they would get a one-night room-only ressie at a WDW resort to show the guard at The Fort.
5) Then, they woud cancel the other resort.
. . . they might lose a one-nght charge
. . . or if they cancel 5-days ion advance, there is no charge
6) Yes, people are that cheap, and MANY folks simply try to think of ways to cheat WDW.
. . . we coud write books on guest-cheating
. . . but, that would only give others hints
NOTE: Fortunately, there are RV parks in the area that do rent space for your RV/camper when you move to other reosrts or take a cruise. I live in Clermont, about 20-minutes form WDW, and we have two parks that have such a space-rental service.