There has been at least one 2 story class A motorhome design offered a few years ago. I forget what brand it was, but it had a sleeping loft in the rear section, and I think a sunken living room area below which went down into what would normally be part of the basement space.
Also on the smaller end, my 2002 Safari Trek meets the concept of having a single room, as there is no bedroom, instead it has a powered loft bed that lowers from the living room ceiling. Stopped the bed mid way down allows for sitting headroom on the bed, and enough room for someone to sleep on the sofa below. Some other Safari Trek models expand on this one room concept, in slightly larger sizes with slides, for one that may fit your concept a bit more take a look at a 2004-2006 Safari Trek 30PBS, it has a small bathroom behind the drivers seat, no bedroom then a large kitchen and living area with side slide in the rear, there was also an optional rear picture window.