No issues finding diesel in the US, especially off highways (trucks use diesel) and in rural areas where lots of pick-ups and farm equipment use diesel. In inner city areas you might have to drive a few blocks but also available at any highway exit. Availability of diesel is really not a factor. The Sprinter is the sweetest driving/touring cab of any Class B - smooth, comfortable, powerful, big picture window, lots of MB safety and convenience features.
Biggest factor is how you intend to use the van. If like to boondock, power systems are key. If have gear-intensive hobbies, storage will be key. If have more than 2 travelers, seating options are important. etc...
Roadtrek is an extremely valuable brand and, despite the current uncertainties with the newly introduced Erwin Hymer brands, will be around for a long while, though ownership is tbd. I would use that as a negotiating tool for price and an overlapping extended warranty.