The problem with a Class B is the width. In a Sprinter you have 71 inches to work with. If you have a toilet like that Class A on the outside wall you would need approximately 40 inches of overall depth leaving 31 inches for aisle and opposite wall cabinetry. Give up 16" for cabinetry on the opposite side leaves you a 15" aisle. You start to see why you need the extra 18" width of Class Cs. That's why the three current true Class B Sprinters, LTV Free Spirit SS, Winnebago ERA 70C and Coach House Arriva all have bathrooms that extend all the way across at the back of the van. That's why most Bs have toilets facing front or back and not to the side. Since you need stepping in space the depth of the bathroom is going to be about 40" deep by anywhere from 20" to 25" wide. It becomes logical to make a wet bath of the space since if you had a separate shower in line even if you utilized aisle space with the shower you would still nearly duplicate the bathroom space. Considering an extended body Sprinter van has but about 92 square feet behind the cab seats it becomes a precious sacrifice of space for the sake of not having to wipe down a bathroom that by the way in my GWVan Legend gives me more shower space than that Class A shown.
Your tanks are still not going to be any bigger. You still will have to take a "Navy" type shower to conserve water. You still will not have a high pressure spray. You could overcome some of that using full hookup campsites but that defeats the freedom of a B. You would give up a lot of storage and other amenities. With a rear bath as currently being offered you kill the functionality of the back end and you might as well not have back doors. Beds become the first sacrifice such as the too narrow 54" wide beds of the LTV and Winnebago slide outs and the mid-bed design of the Arriva passage choke off, a concept that had pretty much been abandoned by the B converters. Unibodies are pretty much limited to the sliding door width to create a slide out.
The Promaster has an additional 3" at 74" total but that still isn't enough and they don't come long enough which would make the sacrifice of space more dear.
So the question is...is a dry bath worth all the sacrifice or are you really in need of a larger RV type?