Finding a site on the coast in southern FL during snowbird season can be a challenge. Land Yacht Harbor is an Airstream park in Melbourne FL you might try there. We stayed at Silver Lakes RV in Naples for a week in December it was a nice park but not inexpensive.
Here is a link to a thread with FL beach places to stay http://www.airforums.com/forums/f42/...da-130556.html
. Some are in the southern part of the state. If you are retired military most of the Florida Bases have RV parks as well. Lastly if you are willing to go north of I-4 in the central part of the state you may have better luck. Salt Spring Campground in the Ocala National Forest is a nice full hook up campground if you don't mind being about 20 miles from shopping. Half their sites are first come first served.
Hope you find what you are looking for.