Check recreation.gov and type in just "Florida." Then select camping and RVs and it will give you several places near Ocala. A map will be available if you click that option. There are at least three federal (cheap) camping areas that are first-come, first-served. Also, Juniper Springs is available for tomorrow night via reservation.
Also check Florida State Parks around Ocala. There are quite a few with vacancies. Very easy.
You should call wherever you plan to arrive, especially Long Pine Key because nearly all Florida state and federal campground lock up at dusk. And by "lock up" I mean it is a serious gate that you cannot possibly get through. If you call, they will give you the code over the phone if you have a reservation. (I forgot this once when my cousin was dropping me off, but luckily found someone at the entrance booth working late. Sometimes, they will leave a padlock that looks locked but really isn't, so check that if you forget.)