I stayed at a Tennessee park a month or so ago. There wasn't any confusion (for me) on getting settled in. I think the office was not open when I arrived, and I just went to my site and attempted to get myself not too far out of level and had a decent night's stay. I did stop by the office on the way out, but I think that was not strictly speaking necessary.
In general, for state parks in most any state these days, if you have a prepaid reservation and there's no guard shack or open office visible, you merely drive in and plonk yourself down in your reserved site and that's that. There were several state parks I stayed in recently where I never had any interaction with rangers or other state park employees (and the office, etc. did not appear to be opened at any time). A lot of these parks were operating on a reservation-only model, though it's often possible to make a call to the reservation center when you arrive and make one for that night.