Take it FWIW...may not be applicable to pulling a 5th wheel, but my DIRECT experience is worthwhile:
- I have a 2013 6.7 in a F550 platform. It was purchased in, strangely enough, 2013.
- I change (to be direct/clear, either I or my son have the dealership change) the oil every 1500 miles and all other fluids at 1/2 the recommended interval;
- I have 22,000 trouble free miles on this engine/truck/platform.
"22,000 doesn't mean much"....true enough, but the 550 is used for snowplowing and landscaping....so that 22,000 is pretty hard on a truck, probably harder on the drivetrain than just the engine itself, but I've had not trouble with the 2013 6.7...or the F550 in general so far.