I would keep your reservation if I were you. I like the friendly atmosphere there (and it is the only Keys state park campground with a laundromat, by the way). You can take their glass bottom boat trip out to the reef, and there's a nice water sports gift shop.
Our first time Keys camping, we based ourselves there and drove south for a couple of day trips. You could go down to Sombrero for a pet friendly beach on Marathon Key.
There are many restaurants, groceries, etc nearby Pennekamp since Key Largo is a well serviced community.
But if you cancel, post on the forum when you are going to pull the trigger, and somebody here will jump on it.