In reply to compost:
I don't think anybody will mind you leaving a car in the car park near the Arrochar & Tarbet station, outside the Slanj restaurant. Only risk is it might be full of camper vans already, but I reckon you would find space.
At Inveruglas you would have to abandon your car in one of the occasional pullouts off the A82, which would be a gamble (they might well be full, and I wouldn't be sure the car would be safe to be left).