Is it the supercharged "petrol" V6, or the TDI engine? That would make a difference, though I'm sure the supercharged engine is no slouch.
If you're looking at vintage trailers under 20' there are a number of trailers with gross weights under 4000 lb. For example, the '70s Argosy Minuet 20-foot models had a gross loaded weight rating of about 3500 lb, with claimed dry weights around 2500 lb.
On the 2007 specs (located
here at Airstream's site, may be useful for you) it shows the max gross weight of a 16' International at 3500 lb, and I know people who comfortably tow one of those with a BMW X5 3.0l.
An option for new trailers might be the Sport line (in the US at least, I'm not sure about their offerings for Europe.) Max gross for a 22' Sport is 4500 lb.