As an alternative, in a month or two, Ford's Transit van will be hitting dealer showrooms in both gasser and diesel models. I'd consider looking at that as an alternative once RV makers start upfitting that. As far as I've read, the Ford Transit I-5 diesel engine will take B20 fuel without issue and be covered under warranty.
Reading MB's position, it seems to do an effective tap dance. They say that B20 is to be used if there is no other alternative, but they don't state that anything above B5 won't void the warranty... and the difference between "you can use B20" versus "using B20 will not void your warranty" are worlds apart. In theory, it shouldn't matter due to the Magnuson-Moss Warranty Act, but this would be an uphill battle to fight.
I'd take a look at the Travato, which might be a decent alternative, and has a very attractive price.
As another option, I'd look at what Sportsmobile can do with a custom floor plan.