A few of the campgrounds we frequent have a rule that the site must be physically occupied each night. A camp host verifies this each night or early morning. A tow vehicle, or in the case of a tent, a car must be at the site.
I suppose a trailer and tow vehicle could occupy the site and not have anyone actually there. The camp host wouldn't be able to determine that, although it would look pretty odd that there is no activity at the site all day or evening.