Good suggestion. I’ve added service:vehicle:sales=caravan service:vehicle:rental=caravan in https://www.openstreetmap.org/changeset/169604647

I thought about service:caravan:sales=yes (etc.) but that would have broken ID’s interpretation of the data. Refining the value of the tag seemed like a more backwards compatible change.

If a car dealer is also selling caravans then the dealership could be tagged with service:vehicle:sales=motorcar;caravan.