Looking for a custom Sprinter RV Shop. Advice?

There is also Sportsmobile in Austin. I have seen other RV places, but they do a very good job of upfitting.

For a "B", I'd say Sportsmobile hands down, although Davydd has had excellent experience with Advanced RV.

For a B+, which is really a chassis based motorhome, I'd probably look at Nexus RV or Phoenix Cruiser.