Going to Keys (not sure which one yet, help)for maybe 6 or 7 days in October. Considering pulling travel trailer from Michigan. Looking to do some snorkeling and fishing. Any advice on campgrounds or which Key would be better access to the area. Or would it be easier to just fly down and rent a car?

We camped at Pennekamp State Park, which is in Key Largo. Nice place, with easy access to snorkeling, both shore and off a boat.
http://www.pennekamppark.com/index.html

Your best bet may be to fly down and rent a car. It'll still be low season--lodging should be plentiful and cheap. It's a long, long way to tow a trailer--Florida is a deceptive state. Admittedly, there are some beautiful places to camp in the Keys, but flying is the way to go if you've got time constraints.

I grew up in Michigan and my family camped (pulled a trailer) to Florida at least once per year.

Our favorite spot in the keys was Bahia Honda State Park on Bahia Honda Key. It's beautiful, lots of fishing. And, relatively central.

It was hard to get reservations, but that was years ago, and we also went in the spring (a more popular time). So you might be in luck.

I LOVE the campgrounds in Key Largo.....

My husband and I have camped several times at the Big Pine Key Fishing Lodge(Big Pine Key) - it's a great spot owned by a local family. Camping right on the water is great and there is a Key Deer preserve adjacent to the grounds. There are nice, clean bathrooms - a great pool - and folks share post their catch each day up on a community board (how great is fresh fish by a campfire?). Best of luck.