Hi Brent,
Not sure about the extra cost, but they have a policy of one vehicle per site. It is loosely enforced, but the sites really aren't big enough for two in most cases. If you park it on the grass, they will leave you a note to please move it. Also, and this if from personal experience, some of the loops can get pretty tight, and when you are trying to back into a site with a camper those second cars actually cause a lot of trouble for some campers.
Is it an option for one of the cars to stay parked in the overflow lot? This is located at the main entrance. If so, I don't think they charge anything for parking there that I am aware of. I did see several cabins with two cars parked there, but they were on the concrete pad.