Can't help you much with Universal, but we know the Keys .....
The Key West Bus line runs a regular schedule from Key West all the way up the Keys to Marathon, so any campground from Marathon to Key West will work for you. Bad news is that reservations are tough during the winter season, so you need to see what you can find for the time period you are looking for. There are always last minute cancellations, but I would not want to count on that happening. We are pretty familiar with all the campgrounds in the Keys, and obviously some are better than others, but none are really bad .....
