Long Key State Park has primitive walk-in sites which are around $10 (I think they are 8 bucks). You can't reserve them and they are not listed on the parks official website. It's the luck of the draw if one available when you ride past. They were full when I rode by, but I would've stayed for a week if one was open.

I think stealth camping is very possible, however, I wouldn't even attempt to camp any closer than mile marker 10 or 12 (Key West is mile marker 0). I am a dedicated stealth camper and I checked out a bunch of places from Key Largo all the way to KW.

Homeless people are abundant in the Keys... which means that good places close to the bigger towns (Key West, Marathon, Islamorada) have permanent residents living there. I checked out places to stealth camp and found I could probably camp in the Keys without much problem. My advice is to use a quality hammock (with a bug net) because the ground is a little mushy in some good places, camp no closer to Key West than mile marker 10, arrive at camp at sunset and depart at sunrise.