Ford is much more aggressive in their fleet marketing, always has been. Look at what they have done in the police market with the Ford Taurus and it's "station wagon" variant, the Explorer. They also build more subtle variations in options etc. The GM chassis is a better handling, better riding, more comfortable, quieter chassis, along with one of the best, least problematic V8's still in existence. The 4500 GM chassis is superior to either the 350 or 450 E series except in GVWR in the case of the 450.
It's what happens when you are the low bidder, & will build to suit way back when the 2 chassis started competing in the RV & public safety market.
Our local EMS started using the GM chassis about 3 years ago when Ford discontinued the E series with a diesel. Now using the Chevy variant with the Duramax, 6spd heavy duty Hydramatic combo, they are having less service repair shop visits....
Concerning the RV market, makers will stay with the same old tired thing until the buying public demands better, and/or Ford discontinues the E chassis