I had a great time there, February 2017. That was before hurricane Irma, so anything I say might be wrong now.
At that time, the state parks in the Keys were often full. We stayed at state parks there, my touring partner lined up the parks. One of the parks, they let us camp in the picnic area but we could not set up until evening and had to be out by early morning. My touring partner did a great job of talking to state park personnel before the trip so we knew where we could get in.
There was a big RV park in Key West, they charged us about $100 for one night in a tiny little RV space that was half used for a big dumpster.
The photo below is a campground on Key Largo we stayed at, they were less than $50 if my memory is correct.
Don't be afraid of the bridges, you get used to them. But this would be a bad place to have a flat. There is a lot of debris on the shoulders.
We did Everglades before the Keys, I really enjoyed that too.
We started in Marco Island, went through Big Cyprus, then down to Everglades, and last the Keys. Then a ferry back to Marco Island which is where our vehicle was. I left my GPS on for the ferry ride, so that is why it is plotted on the map.
I took the photo below standing on a bridge on the highway through Big Cyprus. Lots of wildlife.
Be ready for hot and humid weather. My touring partner had some difficulty with the heat.