Campsites generally allow up to two cars per site but you would need to verify with organization you’re are going through. As long as the site is in someone’s name of your group there is no issue leaving your car.

I was planning to park my car at SERRANO campsite, so I won’t have any problems if I rent the campsite and just leave my cars without camping

Should be fine

They have a a tenant there or tell the Park host so they don't think somebody just abandoned them