I looked under my 06 RAM 3500 dually this morning. Everything looks good. I wanted to check after carrying the new AF 990 to Alaska this summer. That camper is lots heavier than my Northstar 8.5 Arrow.
I tried to get some info and measurements of the RAM frame. I'm not sure if "fully boxed" is the right term but the frame from front to back is fully enclosed. Appears like a tube compressed on the sides to form a beam with rounded tops and bottoms. The sections are about 8 inches tall and 2.8 inches wide. It was difficult to get a precise thickness but it appears to be about 0.2 inches thick.