There are several Class A motorhomes with bunk beds now. Try looking at the Winnebago Sightseer, Itasca Sunover, Damon Daybreak, National SurfSide and Newmar Canyon Star (all gas) as well as the Newmar All Star with three mid-engine diesel models.
Also, in one post somewhere somebody mentioned that Georgetown and Hurricane have bunk models but I haven't looked as yet. There may be more but this is all my research has found thus far.
We have two small children and are hoping to full-time for a year (or more if we can stand it) once my husband retires from the air force next year, so we're finding this very exciting.