Florida keys in Sept?

Expect hurricane.

Keep in mind Sept is the height of hurricane season. The Keys is not a place you want to be if a storm should wander through. And evacuating is a nightmare. That said, you will probably be fine. Weather is nice that time of year.

We go down several times a year. September will be warm/hot and will likely have daily rains.... usually last a short time. September is at or near the peak of hurricane season so who knows what will pop up.
Site 77 is a very nice bayside lot. The tiki has wind curtains, which you will need as the bayside lots are very windy. Great place , you wil enjoy it. It's a short drive to key west ( 14 miles) so it's an easy ride for entertainment,dinner/etc.

Take the Florida Turnpike (pick it up in Orlando) all the way to the end. It drops you off almost on US-1. Do not take I-95 south of West Palm. Do not drive into Miami/Ft Lauderdale.

Follow US 1 south to your destination.

The Keys in September are still in summer, mid-80's to 90's with a sea breeze. Afternoon showers. Be sure to go out on one of the dive boats and go snokeling on the reefs. They'll have all the gear you need.

Weather.com has all the historic and forecast info you could ask for. I looked at their website and WOW, that place is sweet. Enjoy!

http://www.bluewaterkey.com/rvlots/lot_77_bay_front_platinum.html

Went to the Keys in February. It will be my last trip there. I lived in Fort Lauderdale in the 70's. 95 was two lanes each way, traffic is now 6 lanes each way, bumper to bumper, 5mph at 3AM. Never again. You can have your Keys.