There is a caravan park at Wave Rock but there is no free camping there. There is a $10 entry fee per vehicle to park at Wave Rock, if you stay at the caravan park the entry fee is included in the site fee.
If you are heading there across the Nullabor there is a good free camping spot at Pildappa Rock which is a wave type rock formation similar to the Hyden Wave rock which is just north of Minnipa between Kimba and Ceduna that I feel was just as good as the Hyden Wave rock but without the crowd or entry fee.
There is a car park about one kilometer from Wave Rock, I have seen the odd caravan in there, but unable to say if it is a free camp, or if they had stopped for a cuppa
I tried to visit Pildappa Rock twice in the last five years, I never completed the journey on the rough road, going about 30 KPH, perhaps I was unlucky, or went after the Christmas rush
We stayed at Tressies Caravan Park in Kalgarrin just west of Hyden and drove in to Hyden to see Wave Rock. Great park,cheaper than Wave Rock and has a great little museum.