We started making our Florida reservations for this winter while we were still there last winter, including the ones north of I4. Even then we found some of the parks fully booked already, and some of the state parks filled up within seconds of the 11 month reservation window opening. For short term stays of a couple of days or so, you'll probably be okay, but longer stays of a week or more could be problematic. I suggest calling around now to see where you can get in. The
RV Park Reviews website is an excellent source for finding suitable parks with good user histories.