Attached information details why starting in 2016 Model Year, practically all motorhomes that were previously built on E-350 Cutaway or Stripped Chassis switched to E-450 chassis.
Emissions certifications placed frontal-area and weight restrictions on E-350 that did not affect E-450 chassis. All this only applies to Ford chassis with 6.8L V10 engine, which is the only available engine for time being.
It is also interesting to note that for 2017 Model Year, the frontal area and weight restrictions were revised, but the allowable frontal area for an E-350 chassis is probably still not high enough at 72 square feet to cover most motorhomes.
Short-term this seems counterproductive to me because it drives lower fuel economy, but longer term it gives manufacturers an incentive to make new motorhomes with smaller frontal area. Also of interest will be whether other engines (like upcoming Ford 6.2L V8) will have same frontal area and weight restrictions as the current 2-valve 6.8L V10 in E-Series.
While some motorhome owners like having extra payload capacity in an otherwise identical motorhome, it doesn't appear cargo carrying capacity was behind the switch to E-450. Shouldn't manufacturers go back to E-350 if emissions certification permits it?