If you find out you are able to use military camp grounds...some take reservations and some do not.
The last time we were in Key West, Trumbo the Coast Guard base's dry camping area did not take reservations. Sigsbee on the Navy base has full hook-ups and did not take reservations either. What they did at that time was let folks set up at Trumbo and put them on a waiting list. The Sigsbee sites were for short term camping only, I think 2 weeks then if you wanted to stay on at Key West had to go back to Trumbo, back on the waiting list, etc. A lot of people just go into Trumbo and dry camp all season long. They have a nice grassy field and an old sea plane ramp to camp on with a clean bath house and access to a dump and fresh water fill. We stayed for a week and didn't even think about heading over to Sigsbee. We did go over to Sigsbee to do laundry.