Poly fenders, single fenders for each truck axle.

I don't drop the air when unloading, but since this isn't a big operation, the air is often out of the bags by the time we dump on the trailer with the grain cart.   I mounted the finders so they pretty much sit on top of the tires when the bags are empty and the axles are against the stops.    I didn't change anything on the 5th wheel and think it's standard height.   No problems.   I put fenders on because i was worried about throwing a rock threw the back window of truck.

Here is a link to talk with some of my pictures.
http://talk.newagtalk.com/forums/thread-view.asp?tid=400249&posts=21#M3184406
 

For abusive environments, Minimizers or Fleet Engineers poly fenders. Stainless fenders look the best and last the longest providing nothing hits them.  Painted to match Fiberglass double hump or individual fenders look even better but are very fragile. Some of the styles look dumb but to each their own.  Minimizer does offer chrome poly which looks surprisingly good but is very expensive. The single black minimizer fenders are like $120 each without mounts and lifetime warranty. Lay a 4x1 across the tires when you install them with the air dropped, that will get the height right above the tires, remember tread depth too. Then install risers for 5th wheel if needed.