Not sure how things are out in OP's neck of the woods, but around here, most state and national parks (I've never camped in a private outfit) allow one vehicle per individual campsite, maybe a few for a group campsite, with an "overflow" parking lot somewhere for any extra vehicles. I had assumed that one of OP's relatives wants to park a single car in his unused car spot, instead of in a more remote location. Not something that, under those particular circumstances, I'd probably get too fired up about (unless maybe my relative was going to engulf my site with a huge dual-axle, full-crew-cab, long-bed pick-up, or a giant Hummer, or something like that.)

Re: RVs and monster tents and generators and portable kitchens and the like . . . meh, I'm happy to see people at least mininally enjoying the outdoors, rather than being parked at the mall or wherever, but it's definitely not my thing. I always seek out "tent-only" sections of campgrounds, so that cuts down on a lot of that "McMansion-away-from-McMansion" effect.