The roof is a little less important than the chassis, I would think. Both are on a 14,050 lb. chassis. The fleetwood, being two feet longer, weighs more and has more space for cargo (cargo weight). Chances are, buyers will load them closer to (or over) chassis specs. One way to mitigate the problem is to lower the tow hitch rating and therefore the tongue weight (350 vs. 500). 150 lbs. may not seem like a lot, but that is an entire person or 20 gallons of fuel or 20 gallons of water or two six volt batteries...
The fleetwood weighs more and therefore has a lower cargo capacity. They are trying to help you not overload the chassis. Another way manufacturers do it is lowering the fresh and waster water tank capacities.