Near the entrance to the circuit at LeSource there's a car parking area on the right just prior to the gas station (across from hotel), we seen many times tents set up in that parking area on the hill. There has also been tents set up in a parking area just to the left of the hotel in a car park area. Very short walk into circuit from this area. There has also been camping just past Fagnes on the hill. Try contacting the circuit directly, this might prove to be of some assistance -
http://www.spa-francorchamps.be/en07..._tourisme2.php. We always stay at the same hotel in Trois Points on the hill. You might try emailing the LMS folks, they have been very helpful to us.
Carol