Richard
Have you considered an OZTent? - http://uk.oztent.com/Oztent%2DShop/
As you will still have the roofrack it can travel on top. It's a 2m long sausage when folded about 25cm in diameter.
I used one in Morocco this April along with another family of six that used two.
Very easy to set up, two to five minutes depending how many guy lines you wish to peg down, it can stand without any.
Full adult standing room for 60% of area, excellent ventilation and mesh screens and an awing to the front that can have sides and end added, doubling the available space for shade or rain.
They also have a mesh inner for the awning giving bug screening if required.
You would have plenty of room for two adults on camp beds and a child travel cot in versions RV3,4 & 5.
John