We are talking only SIX inches between those "narrow" and the wider ones you mention. Comparing the Thor Magnitude which is a large 35 foot Super C and a Coachman Freelander 21QB (the midget RV on the ford transit chassis), the difference is still only eight inches. As widths go, even a large three inches on each side means one heck of a big difference in interior floor space, but not so much drive a driving standpoint.
Additionally, some chassis have limited wheel tracks that means they can't exceed a certain width of the body. Since the Ford (excluding transit) and Chevy have wide wheel tracks their bodies can be made wider. Step into a Gulfstream BT Crusier 5210 and it will feel and appear larger than a Winnebago Era even though they have the same floor space. The reason is the few extra inches in width of the BT Cruiser makes up for the few less inches in the length.
When discussing "Sprinter" vans, remember the RV versions with bodies are not the same width as the straight unibody versions popular as Class B's. The Prisim on the RV Sprinter chassis is still almost TWO feet wider than the ERA in a unibody Sprinter chassis.
