Bailey alicanto grande lisbon

It's still going to weigh the same on the roof, so you are still constrained by payload whether van or car. A roof box merely adds to it.

Anything on the roof of the car will increase fuel consumption as you have additional wind resistance on top of the big box behind the tow vehicle

We got rid of a Bailey Unicorn after experiencing a so called 'broken axle'. In fairness Bailey repaired it under warranty but it made us realise how pathetic payloads of most caravans are. We bought a Coachman VIP which we were able to upgrade to a higher limit. I have 'banged on' previously on this website about the stupidity of all the storage space in caravans which you can't use because of pitiful payloads. The Club's magazine is complicit because its caravan reviews never point out this anomoly

This was one of the reasons we bought the Knaus which has a more sensible payload of 365kg.  I took our previous Barcelona to the weigh-bridge and with just the basics in it - no clothes or food - it was 20kg over the plated limit. 

Fitting a Safefill gas bottle and carrying heavy things like the Alko locks and EHU lead in the car where measures I had to take to reduce weight in the van.

As for the roof-box - it will definitely increase fuel consumption.  I have a spare wheel for the car on a roof-rack and since fitting it, fuel consumption towing has increased from 24mpg to often less than 20mpg. (3 litre V6 Touareg towing a 2000kg van).