There is a reason that the largest percentage of Class B's are on the Mercedes Sprinter. It has been a fairly easy platform to build on and has the longest track record of the modern style vans. Most of the Rv manufacturers won't even build on the Ford Transit. Roadtrek refuses to build on a Transit and uses the Promaster for their low cost models. Advanced RV refuses to build on a Transit or Promaster. Companies like Sportsmobile will build on any platform and the majority of their customers choose the Sprinter. Personally, I can't sit on the Promaster seats for more than about an hour. I wouldn't mind owning a Transit although the high roof model looks like it was a complete afterthought.
There are about 700 Freightliner dealers across the country that are owned and operated by the Mercedes that are happy to work on them.
There are other significant advantages to a diesel RV. The Sprinter is designed to accept a second alternator which eliminates the need for a gas generator (and all the hassles of a generator). It is also fairly inexpensive to add an Espar heater to use diesel fuel for heating the RV to eliminate or reduce the need for propane.
If one eliminates the Sprinter, you have eliminated a large percentage of the Class B choices and probably most of the best and highest quality choices.