My understanding is this-
There are two camping areas at the Devil's Fork State Park, the regular campground, and the boat in camping at Double Springs, across the lake from the campground.
The area outside the park is the managed by the Andrew Pickens Ranger district of the Sumter National Forest on the SC side and the Nantahala National Forest on the NC side. You can camp anywhere in the Andrew Pickens district but only designated spots in the Nantahala. The Toxaway area is just over the line in NC but those are designated spots.
"The Andrew Pickens Ranger District has a wide range of camping opportunities from developed campgrounds to primitive sites.
Primitive camping is allowed at least 50 feet from the trail, 50 feet from any stream and .25 mile from the road."
Andrew Pickens Ranger District
Phone: (864) 638-9568
FWIW, there are plenty of stealth sites around, but it might be worth a phone call.