There are two reasons:

  • practical: like you said, a vehicle plus towed caravan can hardly accomodate in a 5-7 meter parking space

  • legal: a towed caravan, once stabilized, loses its moving status and become camping, which in Italy is forbidden outside caravan sites.

Note that in several countries even an overnight stop of a camper is illegal. Instead, in Italy you can: that’s why I think is important to differentiate tags for OSM.