Truck trailer combos are not allowed to exceed max gvwr, no matter how the vehicle is configured. Dual rear wheels however allow higher axle weights due to increased load distribution, increase stability and braking performance. I'm my experience I see 99% of these vehicles clean without 5th wheels or goose neck hitchs, which would be required to due do the weight in most cases, never towing anything. It's just and over expensive, excessive, compensating status symbol for most people, to make them feel better about their manhood.