The entire month of March is crazy busy in south Florida. It'll be tough to get a prime spot. Bahia Honda State Park campground is very popular as is the KOA on Sugarloaf Key - maybe one of the campgrounds will have a cancellation. Good Luck!
BTW, we "accidentally" camp in chilly weather - this year so far the low has been 26 at night (and the state park water pedestal froze). We have a 4 Season package on our Lance trailer, which includes enclosed heated tanks and dual pane windows. We haven't had any trouble, although the campgrounds usually recommend you detach your water hose at night if it's likely to freeze. Also, make sure you have enough propane & battery charge so the furnace will keep you warm all night - ask me how we know! :)
