I would say if it is left stock, and you dont try to use it like a Semi tractor, you should be fine with what you are wanting to do..... I have towed heavy with mine a couple of times and was not worried that I was going to hurt anything. A 30-35 ft 5th wheel, even a toy hauler would be in the neighborhood of 10-12k I would think. This should be within the limits of a 350 with out trouble... my Ex, which is a 250 equivelent, has an 11k trailer limit. I think the key is not to turn the powe up on them.... Ford has them tuned to about the max safe limit from the factory. Keep this in mind, A 6.0 PSD from Ford is 325hp/560lbft and a redline of ~4200 with the factory programming, but the IH (Navistar VT365) I see references for bus equipped to be 175-215 hp and in the MXT a 305hp version. So, IH to have the longevity from the engine keeps the hp much lower and has a redline of something like 2800..... similar to the IDI pumps that the Ford spec pumps have a 3800 govenor and an IH spec pump is 2800..... Just some things to think about....
BTW - they do that same with the DT466....... it is held ot something like 250hp....... when we all know they can be reliable at 600+