You will find that just about every site is booked 11 months in advance...there are some walk-ins at Long Key, Curry Hammock, and Bahia Honda but if not you can sometimes find a private sit nearby..usually $5o plus a night. Try : Larry & Penny Thompson Park in Miami...although booked up they always have dry camping
sites available. Also try Fishermans Cove( Camp??) up near Big Pine key...also usually dry/ overflow camping.
If all else fails, there are some Mom& Pop motels north of Marathon for $80 ++ which are on the ocean.
Some campers try setting up on the parking areas on the bridges...and pretend they are all-night fishermen...some get awa with it.
There is NO Walmart / Big K camping in the Keys!!! Cops'll roust you!
If you have a tent, Long Key S P has a boardwalk camping set-up with lean-to's and ramadas for tenters.
Voice of experience talking here ...we will be in the Keys for Feb through March. Takes some diligent work on the computer to get these ressies.
Final thought...some people double & triple book and "dump" their extra sites a few days before they finalize. Keep checking Reserve Americ right up to the day you want to camp...you could luck in.
Alistair & Inge in the Road Toad (currently camping in W P Franklin CG in Fort Myers)