Often it's more the chassis type than anything else. Front engined Class A's have a fairly long chassis section cantilevered back from the rear axle, carrying all of the load of the "house" so there isn't that much strength for 1,000 - 1,500 lbs of tongue weight as you might get with a 10,000 lb trailer, not to mention what that might do to front axle weight. Rear engined Class A's have a completely different type of chassis, designed to carry the high weight of the engine and have sufficient capacity for the larger load. Some high end rear engined Class A's are rated to 20,000 lbs trailer weight (some Prevosts)
Brian
2005 F250 Supercab, Powerstroke, 5 speed automatic, 3.73 gears.
20 ft race car hauler, Lola T440 Formula Ford, NTM MK4 Sports Racer
1980 MCI MC-5C highway coach conversion
2004 Travelhawk 8' Truck Camper