Margaritaville resort now has a campground open. $85 a night is still pricey, but it is roughly half of the Fort. Looks like they have golf cart rental...not sure if private carts are allowed. The water park and beach area looks pretty nice, and you can actually get in the water!
Edit...just called them. Private carts are allowed and you can take them from the campground down to the water park and restaurant areas.
https://www.lanierislands.com/margaritaville