Even though Dana spec's their D80 at 11K lbs, it's easy to get to that weight with one of these big campers that has a lot of storage and big tanks. I don't think the AAM axle is rated for much more, so changing brand of truck will not help you - You need to change the class of truck you are looking to use.
I'm convinced the F450 pickup has a neutered suspension to increase ride quality. This is why I suggested going to a real Class 4 or higher truck instead of this 'tweener that should be called a Class 3.5 pickup (I give it an extra half over any of the Class 3 DRW's). Perhaps Boatycall will be able to slip a chassis cab suspension under his to get better weight handling, but I know he also is looking at other options that are less payload intensive than his current setup. My Class 5 truck when empty is over 10,000 lbs which gives me close to 9000 lbs of payload - The Class 4 trucks are not much lighter (leaves you with 6000 lbs payload), so don't expect to haul one of these big campers with one while towing a heavy tongue weight trailer.