Over the past year, I've towed with the McKesh mirror for several thousand miles and probably 30 days. Before I put them on, I sometimes (but not always) take a wet paper towel and pipe the spot on the door where the mirror pad will touch. Have never cleaned the pads themselves, though, and there have been zero marks.
You might be a bit frightened when you first put them on about last effects they may cause to your window seal. In the process of putting them on, the deformity looks like it could be permanent. But within a few hours of taking the off, or at most a day depending on temperature, the seals return to their normal shape.
I wish someone made permanent extendable mirrors for my car as they do for some of the Ford and Chevy pickups, but they don't. I've tried a few other solutions, including the CIPA-type that were supposedly made just for the Sequoia, and the McKesh in my opinion are the best option. They don't vibrate, don't have to be continually adjusted, don't move once installed, are easy to install and remove, and most importantly, give the best view of what's behind you. The further out from the vehicle, the better.
You can buy them direct from the manufacturer if you so choose:
http://www.hensleymfg.com/mckeshmirrors.html
Dave