Longer is better for an awning for sun shade--but
not for wind!  A very important consideration is the
mounting of the clip gizmos on the side of the van
for the awning poles.  The rear one has to be behind
the back edge of the sliding door when the door is open.
The length of the awning then determines where the front
one will be.  For a 10' awning, that is about the middle
of the front door.  The awning has a vertical range that
the gizmos should be in.  This will affect your choice of
locations and hence measurements.

An 11'-8" might just fit--sticking out some on the
front and the rear, but you should mark where you will
be mounting the gizmos, measure, and decided.  Having
just quickly measured and compared with my 10', I
would definitely stick with that length myself.

You DO want to mount the gizmos--they make the awning
MUCH more stable in wind.  Ask me how I know; ask Bob
Stevens how he knows.  ;-)  [I call them gizmos; Carefree
of Colorado (my awning) calls them Bottom Bracket Assy
(part #10 at http://tinyurl.com/yvlrzq)--I don't know
what Fiamma calls them.]

Richard
Boulder