Try Long Key State Park in the Florida Keys. It's between Islamorada and Marathon. All 60 sites back up to the ocean. It was 10 feet from the back of our 17FD to the ocean. Sunrises were great. Think it was around $35 per night. There is also good privacy between sites. Also try Curry Hammock State Park about 15 minutes south of Long Key. Most of their sites are also on the water.
Wow, thanks for the tip. We used to go to Bahia Honda and John Pennycamp - by boat. Getting an RV reservation there was difficult as it sold out. One wily Canadian couple we met kept an old boat on a trailer down there - with a blown engine. RV sites in those two campgrounds were scarce, but the boat slips were plentiful. They would launch the boat and Bahia Honda and paddle it to the site. Voila - instant RV.
The last time we were in the keys by RV, they still had the "no pets" rules at Florida State Parks.
Maybe it is time to go back again!