We have camped 6 times in the park over the last 3 years, the last spot we camped at in August was the Langkloof and it was the best by far(although all the campspots are great).
It is only accessible by 4x4 and is an informal camping spot.
But they had just opened a formal camping spot when we were there it is called Perdekloof.So maybe try phoning them or emailing them about it(email address on SAN parks website).
2002 Hilux 2700i(facelift) 4x4 DC
1995 Hilux 2.4 SRX(4x2)
1985 Suzuki DR600S
1984 Honda ATC200ES BIG RED