A mate who installs electric security gates, cctv etc has one of the pull out awnings attached to his van, how often do you have that sort of access on an external install?
I've owned a few of those metal foldup gazebos for the garden, good idea in theory but they take up quite a bit of room when folded and a bit of a lump to have in the back of a van...best bet I'd say is to a look at a Coleman gazebo. Every option will need pegging,