I bought a pair of cheap ones on ebay, the kind with the rubber straps that wrap around the whole mirror, have to say they are fantastic, never come close to falling off, the rubber straps really grip to the mirror, the glass isn't the best in the world, but they don't vibrate much and are perfectly adequate.
I got these before i got my caravan as they cost me about a tenner, I figured i would look out for some better ones once I was started touring but I have decided I don't need to.