Well it stands to reason that it would make sense that the wall coverings found in the Classics are going to provide some additional R value to the walls. Especially the carpet type lower wall covering and the ceiling covering. I've often wondered the same thing regarding whether the bare walls would radiate a tad more of the outside temps.
One thing for sure though, the blinds only or roller window shade type coverings allow much more cold to radiate off of the glass. The double window shades and curtains found in the Classics probably do the best job in minimizing the cold.
'04 Classic 30' S.O.,'03 GMC Savana 2500,'14 Honda CTX 700