A lot of people don't realize that the high power density chassis cab axles (aka Dana 110, 130) have input power limit specifications that are less than that of the Dana 80. I know it doesn't make sense at all, but I've read the paperwork from Dana (can't point you to it, sorry).
This is one of (several) factors that enters into the equation that explains why the late model chassis cabs having engines that are derated when compared to the pick ups.
The F450 pickup is better matched to the 3500 than it would be to a 4500.
The F450 chassis cab would be better matched to the 4500 chassis cab.
I too, wish that a GM 3500 would be thrown in for comparison. I also wish that an actual 5th wheel RV trailer would be used. In particular, a true towing test would include frontal area loading, not just with a billboard facing front with weights on the back of flat bed trailer. A more realistic test would include the fully box sail profile of a real trailer, with all of it's eddy currents and other wind loads