We were told, in 2007 on a trip to Minnesota with another van camper couple that we could not occupy the same site with two "RV" units unless some one slept in a tent. We got two sites, no big deal to us, but a strange rule. We asked if we went to wally world and bought a tent and erected it on our site could we, the two couple in two vans, occupy the same site. The answer was "of course". I don't understand, nor pretend to know.
In MO State Parks the general rule is three roofs, not counting a kitchen fly/table canopy.