Typically a family event like this happens because a few family members spend lots of their own time and effort putting it together for the enjoyment of the group. Unfortunately, they sometimes project their attitude of cooperation and accommodation to other family members. It may be that they simply see the unused parking spots as "available" to the group and assume that the OP would not have a problem with it. Probably the extra parking is needed because various family members are coming from different directions and meeting at the campground; staying with others.

I wonder if anyone is being accommodating or generous to the OP. IOW, is the OP transporting all his own or ratable share of food, beverages, ice, coolers, charcoal, grills, etc.? Or are the "evil" autos being used to accommodate him? If they are, it would seem pretty selfish to be uncooperative.

It's something to think about. Me? I'd allow a few to park, but also express my concern to the planners that I don't want to feel like I'm camping in a used car lot. That would seem reasonable.