I know it wouldn't be overly practical in almost any situation, but I still think it would be pretty cool to do, easy and sensible no, but cool, absolutely.
To hide the windows I had this thought of putting glass in for the ceiling of the trailer and the top and sides of the slides. I work for a glass company so I know how strong glass can be, I also know its really heavy and framing the roof that strong might be difficult, but perhaps doable.
Part of the reason for wanting the custom trailer is because most of the time a semi pulling a regular 5th wheel looks a bit odd, and as a guy that cares about the "street credit" of looking cool, that just cant happen. Otherwise I would totally just get a 5th wheel painted to match the truck and go from there.
Is it sensible and practical, not even close. And thats why I would do it.