My personnel preference is for not staying in Key largo...not my favorite.
Marathon or any of the other state parks. Good luck getting a reservation though...very difficult at best unless it is may or beyond. Don't even think about overnighting in the keys because you will get to pay a $100 fine. We've been to Sunshine key and they pack them close...like and apartment house for campers. We have stayed in Jolly Roger travel park and liked that one better because it is smaller yet still convenient enough to Marathon.
Figure you are going to pay $75 to maybe $100 per night...yikes!
Donna