When I went shopping for a small Class C and discovered that some of them were offered on the heavier duty Ford cutaway van chassis, that was where I focused my search. Here's why: My "tool philosophy" was to always as much as possible for best performance use a heavy duty tool in a light duty way. At the time, I didn't know the specifics as to the differences between the two chassis, but I trusted Ford to have done some things different between the two chassis and that those differences would mean that an E450 chassis loaded well below it's maximum design level would in the long run be superior to an E350 chassis loaded much closer to it's design maximum.
We have had a ~11,800 lb. 24 foot Class C on the E450 chassis for around 13 years, and it has provided what you would expect when underloaded by the coach structure and it's contents -> need brake pad replacements way less often, pull harder on grades due to it's higher gear ratio rear differential, and due to this higher ratio not slip it's transmission torque converter as much (to reduce transmission heating) when crawling off-highway with a heavy vehicle, sit a bit higher and more level all around (I also use larger diameter tires for improved ground clearance of all drive system and suspension system components), not sway as much on highway curves and when entering/leaving parking lots, not tend to get pushed sideways as much when a big rig passes by, and ... most of all ... reduce to zero any concern of ours on how much or in what way we load it with gear. i.e. We can travel with all tanks full and it's handling is not affected to any extent.
Yes ... it used to ride stiff in the rear due to it's E450 leaf springs back there (the E450 front coil springs provide a decent ride in the front). However, I took the sting out of the ride in the rear by using rear shocks that do not add shock stiffness to rear spring stiffness on highway potholes and cracks. You can do this with special shocks in the rear that provide soft - or no - damping on fast road surface changes, but stiff damping on gradual road surface changes. One type of variable action shock that does this is Koni's FSD shocks, which is what I run in the rear.
Note that most of what I say above DOES NOT apply to a larger/heavier Class C motorhome on the E450 chassis - which is the chassis that is absolutely required for those. I'm only addressing why I chose, and my experiences with, a smaller Class C motorhome built on the E450 chassis which could have been built on the E350 chassis.
The bottom line is, though, that use of an overkill chassis under a motorhome may ultimately depend upon your "tool philosophy". ;)
