Why not look at getting a custom tent made to suit your own trailer
Theres a thread on here from Johnny Trackabout, something like 'What an amazing trailer', where a customer built the trailer, took it to Trackabout to get the tent made up to suit the trailer.
Its another option to limiting yourself to off the shelf tents.
Also think about how much time you spend in the tent.. is it just to sleep, get changed, then your out for the day?? 9ft should suffice for that..?
Remember, 12ft tents can be tight/wont (from what I've seen in some) fit on some caravan park sites if thats ever an issue.