When we bought the truck we swapped with a salvage yard for the highest 5th wheel we could get. There is approx. 3" between the fenders and tires, and another 3" between the fenders and trailers. The trailers will hit the fenders every now and then and you can see where the tires have rubbed the fenders underneath, but they've never been torn off.

We used plastic Minimizer mounts on that entire rig. They won't work for a grain trailer (Timpte at least) with dual wheels though. The mounts are the most expensive part. I picked up a whole slug of blemish fenders for $50 a piece back in '06. When they say blemish they mean a nick in one edge or a scratch that can easily be hidden on a working truck by placing it on the inside.