Don't you mean the frame weighs 634? I would think the shell would have the roof, sides, doors, windows, etcWeigh up time. We use Solidworks modeling software to draw and design our campers. Solidworks provides a pretty good estimate of weight once all the material densities are placed.
Our drawing shows our camper shell to weigh 646 pounds. Placing it on a recently calibrated scale shows the shell actually weighs in at 634 pounds. This includes a 1/4" thick floor of aluminum.