Where can we park a second car for our family who arrived in a car and a RV towed by a pickup?

There's plenty of room for a second car.

Plenty of room. This is one of the best state park campgrounds I have ever seen.

You are allowed to have two vehicles on your site if there is room.

There's a massive parking lot at the end of the road that goes into the State Park. Keep going past the camping area and the visitors center and you will find it. Should be plenty of space for you to park multiple vehicles and probably an RV.