Towing a caravan with a roof box on the car.

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I'd have thought the affect of the roof box will pale into insignificance compared to the big white wobbly box you're towing. 😉

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We did use a fixed rigid one when the kids were smaller and came with us on holiday. There was no difference as I could tell at all in towing with or without. Probably less MPG but I never counted.

There was more wind noise and depending on your car (we had a tall 7 seven seater) it may mean you can't get into car parks with height barriers when solo. 

There are fold up removable one by Thule (and other makes) like this:

https://www.thule.com/en-gb/cargo-carrier/car-top-carrier/thule-ranger-90-_-601100

 

Just remember that your roof will have a weight limit and more space means more weight overall.

 

I cannot stand roof boxes. By the time you have the weight of the bars and the box, most cars can't take much in the way of luggage.

 

Though most people ignore any and all weight limits.

I've used one for the last few years - been across to France, Netherlands, Germany and Austria.  Does not have a notable effect on performance.

We always take it off once on site, just in case we end up in a multi story car park.

I tend to put the awning in it so its out of the way as don' t normally put this up until the day after arrival on site - all sits well within the max weight for the roof.  

Would recommend getting one that opens both sides for ease of loading.

Cheers - Dan

As MM says compared to the van likely to be minimal effect. As suggest3d would go for one that can be opened either side and get as good as you can afford.
Our original Thule box, passed on to my daughter, is still in great condition after 18 years of regular use.

We don’t use the box for anything heavy, it cuts down weight and is easier to load, but when carrying passengers especially it can make a lot of difference.

I have a Thule that opens from the rear so lid pushes up and then you have good access side and rear, handy to stand on tow ball and push things in, always put awning inside, dont like to be top heavy, only downside for me if travelling solo, is wind noise once you hit the legal limits on motorways, but not too bad at 60mph when towing.