Towing with a convertible...

What tow limit do Mercedes give? 

Assuming your  E-Class convertible is type approved for towing it will not be a problem, all my tow cars in the 70's and early 80's were convertibles.

Mercedes say a limit of 1800kgs...

Towing at 100% is not ideal if you are inexperienced however you would not be breaking the law. Ideally you should consider a heavier vehicle or a lighter van.

Just a passing thought.

The heavier the caravan, the heavier the nose weight on the towbar which can be a limiting factor. ( 7% of Cvan, weight? I am not a tugger  so someone will confirm)

From memory, many years ago, our van became very skittish when the nose weight was too light.

rgds

In the case of the Adora Isonzo 7% would probably be more than the weight limit on the van coupling anyway which is usually 100kg. Not quite sure why you think nose weight would be any more of a problem when towing with a convertible though.

We used to tow with the OH Focus CC and the steel folding roof and the mechanical operating  gubbins makes for a heavy car especially when compared with the standard 2ltr Focus.

I once sold some Saab accessories to a guy who towed with his 9-3 cabrio without any problems. As a result I seriously considered an Audi A4 TDi Quattro cabrio but the lack of boot space (with hood down) and no5 keen on carrying our dog on the back seat curtailed that idea. 

Having owned a Ford escort convertible many years ago, you could tell the body twist if you ran over a match.  

Put a caravan on the back, I would imagine the engine and gearbox would take off and leave the rear half of the car sitting on the road.

design has improved , I think 

rgds

Wife’s car is a 2009 model.  The real downside of towing the van was the 18mpg