Hi Tiggs
we use towing mirrors by milenco, which are called milenco aero mirrors, they clip onto the plastic mirror casing and give a very good vibration free view. will post link http://www.milenco.com/mirror-aero.html
Have you try'd towing without them?,i tow a caravan with my dobby cargo and with the van being quite wide the mirrors work for me,plus mirror extensions make the van a bit wider and i know the roads are a little narrower down in the depths of Wales ;o)
Oh i did fit blind spot mirrors inside my dob's mirrors which also helpt,got them off ebay for about £2 inc postage from japan.