I think all class C roofs, no matter what the material, would qualify as one-piece. I would be shocked if any maker pieced together multiple pieces of material for the roof covering these days. (That's not to say that, for instance, the front cabover section and the roof are a continuous piece without a seam.)
There are differences in how thick the fiberglass is, but finding out that information between manufacturers is tricky at best. There are also differences in how the edges are sealed and assembled to the walls, and possibly in whether the roof is crowned at all or perfectly flat (crowned, at least slightly, being decidedly better).