I used to have a high-top motor caravan (a Toyota Newlander) that, at around 8'6" high, was too tall for local garages, so I used to have it MOT'd at the local HGV MOT station, where they had a day for testing that class of vehicle once a fortnight. The vehicle was built on the Toyota Hi-Ace pickup truck chassis, and as part of the test, the tester inspected the interior to check that beds were present. He explained that, if it hadn't had beds, he couldn't have tested it as a motor caravan, and it would have had to have had the more stringent test applicable to a commercial vehicle. So on that criterion, the essential difference between a motor caravan and a van, is that one has beds and the other does not.
The Camping and Caravanning Club administers various minimum facility sites. Some are licenced only for tents and not for use by caravans or motor caravans. In response to a query that was published in their magazine some years ago, it was confirmed that motor caravans were allowed on such sites, as long as you also had a tent and slept in that.