it's crazy busy at the parks. MK is open till 1am, and i'm here at the lodge and it's pretty full, with the parking lot and here in the lobby. Usually by this time of night it's pretty quiet. So i don't think there will be much luck at the fort.
There is a KOA off 192, just a bit east of 535, but most people think the turkey lake campgrounds are nice up by universal.