Yes, it is very hard to find a campsite in the keys in season, but a friend of mine just got three nights in Marathon and three nights in Key West (Boyd's) on short notice. People sometimes cancel their reservations, and if you are lucky you can get a break. But DON'T leave the Miami area for the keys unless you have a confirmed reservation. There are zero boondocking or overflow parking areas in the keys. They are very strict about this and for good reason as the land is just too limited to allow it.
Marc and Jill, Wellington FL
2013 Entegra Anthem 44SL