As a 20 year resident of the Keys, I am unaware of anywhere you can legally park an RV overnight and stay in it. While the Key West spot and the Islamorada library might work, it's only because campers were overlooked by the KWPD ( Key West Police Department ) or the MCSO ( Monroe County Sheriff's Office ). It's just as illegal in those spots, it's just more easily overlooked. The point is, AFAIK, there are no places in the keys were you can legally boondock like you can on some public lands.
Frankly, this is pretty understandable. In season, demand for camping spots is pretty high. For some events like Fantasy Fest or New Years Day, if you could camp in the Kmart and Sears parking lots in Key West there would be no room for shoppers. This is also understandable, considering that the in-season rate for a RV site with water and electricity at Boyd's campground
is $80/night and during events some places require a 7 day stay.
There are no Wal-marts in the Keys ( the closest is in Homestead, just south of Miami ) and few big box stores. Those that are here keep an eye on their parking lots. You're welcome to chill in their lot if you're shopping in their store, but don't stay overnight.
When I first got here in '91, you could sleep in a vehicle in the bridge parking lots. Not legally, I'm sure, but it was tolerated to some limited extent. I did a lot of that in my '76 VW van, pulled up next to friends with larger RV's. By '92 that was essentially done, but I had moved down to Key West and was sleeping in my van on private property by then.
Some of the bridges in the upper keys are used for fishing and there are sometimes RV's parked there at night, so it might be possible to hang out for a night or two, but I can't say that I've done it ( I've got a shrimp net, perhaps I ought to give a try! ). I've got no suggestions for the lower keys, most of the bridges down here are different.
The practical upshot of this is: you're probably out of luck for boondocking in the Keys, so don't depend on it. There might be somewhere you can hide and get away with it, but the Keys are not Quartzsite. I'd suggest a reservation at a park. Bahia Honda
is really nice, but book now because the next available dates for RV sites are in May 2013.
Eric - Big Pine Key, FL
2003 Holiday Rambler 29RBD/2006 Jeep Liberty CRD