There is a large overflow lot to the left as you approach the entrance to the North Campground. I think you could park an extra vehicle there. Although, as I write this, I am unsure if you can leave a vehicle there overnight. Checking with the rangers would be the best option.

When we drove through he campground I did not notice any locations for extra vehicles. Depending on the size of your camper, there may be room at your campsite for another vehicle but I do not know if they will allow one to be present. Your best bet is to call the ranger office for that district. I've found that most of the NP and USFS offices are very helpful when I call.