It's a pretty beefy truck to begin with. Rated for 14k. I'm linking a picture of their build up below. The truck has a 3 point pivot frame on which the camper sits. I'm speculating that a 3 point pivot frame which loads the truck frame at two points in the front and one single point at the center in the back, may contribute to frame flex. I'm interested if any engineer types here agree.
http://www.hackneys.com/mitsu/photos/buildup_album_03/image001.htm
Since the load is concentrated at the two ends of the frame rails it allows the middle section to flex more than if the load were evenly distributed on top of the rails throughout the full length.