In reply to pasbury:
Yes, you can leave a car at the showcaves / campsite for a fee (pay in the cafe usually, or someone on a quad often patrols around). There's also a short section of dead end road on the east side of the A4067 immediately past Tafarn y Garreg (pub on the A road) which seems fine for leaving a car overnight. You could also ask in the pub if you could leave your car in their car park across the road.