As mentioned, Disney's Fort Wilderness is a beautiful campground. We have stayed there a handful of times. It's quite expensive, though.
Here's an older write-up about it:
https://learntorv.com/camping-at-disney-world-fort-wilderness/As for driving down to the Keys, we like to go down to Miami first for a night or two. We have always stayed at Larry & Penny county park. It's nice and has decent spots. It's right behind the Zoo and you'll occasionally hear the lions roar. We do that so we're fresh and awake for the drive down through the keys and are able to arrive at our campground there earlier in the day.
In the Keys, we've only stayed at Bluewater Key resort. It's absolute luxury (with a price to go with it!). Be aware that December is considered peak season in the Keys and pricing reflects it.