We've stayed up at Twin Lakes a few time over the last year for my DD graduation and wedding. Lake Hartwell came up last year to full pond and has been about there ever since. You can check current level at
http://www.mylakehartwell.com/Level.asp or better yet, the
Army Corp of Engineers where they will even give you predictions 10 weeks out.
Easiest way to get there would be I26-I385-I85-Exit 14 south, left at stop sign (careful, down hill stopping), cross the lake, right on 187, follow signs. This assumes you get to the toll road by 3. If after 3, take I185 (toll) to I85. I usually get off on 153 because I go to Easley, but you may want to do the same if around 4:30 -5 as I85 likes to back up some between I185 and Hwy 153. After 153, it should be hectic but flowing.
If you are backroad adventurous, would run up as I described earlier. Would also give you the chance to swing through some of the parks on Clarks Hill Lake, especially Calhoun Falls SP. Just stay on 187 until you see the sign pointing to Springfield.
As a backup, don't know why, you can try neighboring Saddlers Creek SP. They seem to always have sites available. Guess it has to do with the number of campgrounds on the lakes and no large population centers close by.
2015 Ram 3500 4x4 Crew Cab SRW 6.4 Hemi LB 3.73 (12.4 hand calc avg mpg after 92,000 miles with camper)
2004 Lance 815 (prev: 2004 FW 35'; 1994 TT 30'; Tents)