It all comes down to weight, length and height. The E-450 with the dually configuration allows for a much bigger motorhome than a Transit or Sprinter. And as Bob pointed out, it's about building models they think will address a target market and choosing the best / most economical chassis for that design.
However, just because the E-Series can handle a bigger motorhome design doesn't mean it's ideal.
I believe that any motorhome 25 feet or larger on the E-Series pushes the limits of the stock suspension and results in poor handling and ride quality. When Ford (or any manufacturers build a stripped / cutaway chassis, they have no idea what the purchaser is going to do with it.
The chassis could be a shuttle bus, cargo truck, ambulance, motorhome, etc. Ford use standard tires, springs, shocks, etc. to meet general and not specific applications. It is up to the purchaser of the chassis to upgrade the suspension as necessary for the application. This is something the RV industry does not do for the most part when they design and build their models.
It was my experience with a 30' Class C that the suspension had to be upgraded to get it to handle and ride properly from my perspective. I had to install Hellwig Heavy Duty Sway Bars, Sumo Spring and Roadmaster Steering Stabilizer to get it where it needed to be based on the weight, length and height of the box. It could have also benefited from better shocks but I had already spent $1500 on parts and did the work myself.
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