Ron/Joseph
I do know that MPG is not that great in the ecoboost in a 9500 lb (out of the factory) Class C RV. About 11-13 MPG. The slew of HD 350 Transit B class is getting 13-15 MPG.
Hutch
And our 2007 E350-V10 PC 2350 gets 10.5mpg (a ~5000 mile trip average) when driving it like a TRX or Sprinter (highway speed limited to 62mph) and not towing. My opinion is that 11-13 mpg is not a big enough of a difference, giving up one of the most reliable power trains with lots of power throughout the power band, just to gain a little in fuel economy. If you can assure 13-15 mpg, then..... maybe....but I feel you have to drive a lot to justify it.
I believe Phoenix installs a propane generator in their diesel-driven chassis. I feel that is another "ding". We use our generator. I wouldn't want to deplete critical propane for that purpose. I need as much propane on hand for the fridge, cooking, hot water, and the furnace. If you rarely boondock, then this is a non-issue.
Another point about an E-series with a gasoline generator is that if you live in tornado alley or hurricane country and loose power for days or weeks, you have a 41 gallon gasoline supported mobile power plant that can power your critical at-home operations consistently for roughly 80 hours between fill-ups. Even longer with extra gas stored in your garage.
There are other reasons people want a TRX or Sprinter, but that is another topic.