I have a V70 D5 Sport (55Reg) and regularly tow a caravan.
Initially I had 'Milenco' extension mirrors with screw fixings and rubber straps which while good took a little while to position and adjust. They also had a tendancy to mark the paintwork on the body of the mirrors.
Around 18months ago 'Milenco' improved the clamp fixings to the extent that the rubber straps are no longer required, these mirrors are exellent, very quick to install and if removed carefully can be refitted without further adjustment. See link
http://www.milenco.com/mirror-aero.html. These are widely regarded as some of the best mirrors available and are almost vibration free.
On my 06 model V70 I have found it still possible to operate the electric folding mirrors with the extension mirrors fitted and without these contacting and damaging the bodywork; this can be quite useful on say a ferry, etc. However it is essential the mirror mounting bar is above the door mirror, as on the link photograph, and not below which is an alternative method of securing them.
I hope thats useful info.
Regards
BryanP