While down in the keys by boat last year I found a great new little RV park called Curry Hammock. It is state run and located on Little Crawl Key. The RV park is new and you won't find mention of it . Here is the link to the sate park info.
Main Page - Curry Hammock State Park » Florida State Parks
While we were there, wild dolphins were popping up in the lillte lagoon to greet vistors. A manatee was rumored to live there too and we befriended a few hermit crabs on the beach as well.
Some Canadians that I met who were staying at the RV park said that the place was getting booked up a year in advance since it opened.
This Google satellite image
shows the park under construction.
It is the land mass on the left. You can clearly see the RV sites. The land mass to the right is Valhalla Resort, the last little bit of funky old keys hospitality left. We found it by accident while trying to hole up during a storm on our way to the Bahamas to watch filming of Pirates Of the Carribbean!