Marty,
I would say as soon as you know your exact dates.
However, summer at the Fort is not a busy time like you experienced at Halloween because it's so darn hot/humid/rainy. The holiday weekends are busy (Memorial Day, July 4th, Labor Day) but that's not your time window. Those holiday weekends don't see the load (camper, visitor, or cart rental) that other holiday times do.
If you are camping in July in Florida it is IMPERATIVE to get a shady campsite to take the load off your AC units. And since you're from Wisconsin, I will add for you to bring some SPF 100 and apply it liberally to exposed parts of your body. FW and WDW is doable in summer but it frequently rains about 3pm every day (hard) for about 10-15 minutes then it stops and it's humid for several hours as the rain evaporates and the sun goes down.
But to answer your question, there's no rush - just when you know your dates. At the Fort, summer is a slower period.
Bama Ed