[57tailgater]

I believe you would have to figure out a moment diagram on the whole set up - calculating the loads and seeing which way things want to pivot on the rear axle centerline. That's a very long wheelbase truck plus you have a car up there at the front as well for "counterweight". I don't think you could bring the 5th wheel much further ahead or you would have the car on the truck and first one on the trail hitting each other. The car at the back would be reducing the weight on the 5th wheel as well. A balancing act for sure.