The rural complex is all used for event parking. The circus rolls into town on about Tuesday so unless you get there a week before and cunningly camouflage your camper as part of the building (build a shed around it ?) I think they might spot you.
Back in the day I think a lot of people used to just sleep in their cars along the Glen Nevis road, but I think the local council have clamped down on That Kind Of Thing. The town really does get full, we always book the year before (sorry to be smug).
As people have said, if you head a bit further north you might have some luck.
Also, there is usually a big field of dirty hippies free camping in a field about half a click south of the venue – I don’t know how official this is but it seems to be tolerated. Although ownership of a camper van might make you The Man and they might throw carrots at you.