All of the campgrounds down there are surprisingly expensive. The last time I thought about camping, I just ended up getting a motel for the same price. I have stayed at the Key Lantern on Islamorada. It is dirt cheap, clean, and has a bed and a shower - exactly what I needed. As a bonus, a short walk away is the Lor-E-Lei bar. Great place with their own beach, great views and live music throughout the day. Decent food and reasonable drink prices, as well. Unless you are just dead set on camping, I would consider a motel.

Regardless of how you stay, if you want to fish, I would stick to the middle keys area; Islamorada, Marathon, etc. That area is a lot less crowded and offers excellent fishing just about anywhere you look. Bring a kayak if you have one, or at least rent one for a day. Wherever you are, go into the local bait shop, buy something, and chat up the guy at the counter about the specifics for the area. In my experience, they have been extremely helpful.