We love our parks almost to destruction. The custodians of our great outdoors have to limit the impact of our visits so it will still be the great outdoors when we return. One RV per site is a "bright line" rule that limits impact, is easily interpreted by guests and can be enforced by the host.
Campgrounds sometimes have limits on vehicles or campers specified in the operating permit, zoning permit, sewer permit or by the fire marshal. Those denied entry are not necessarily entitled to the history of the rule.
Every rule starts with an experience that those in charge do not want to repeat. Now we have rules limiting camping at many beaches, BLM lands and Walmart because of the behavior of those before us.
I also tried to double up on a site with my TC once-with no success.