Lyle wrote
"I agree that there won't be a =massive= weight transfer, but there will be =some=, as the king pin is most likely ahead of the rear axle. In a perfect setup, the weight transfer would be near-equal."
Actually, any decent setup should be capable of having the trailer slow down at a greater rate than the tow vehicle. Therefore, there is a minus weight transfer. That is the only way to keep the trailer from pushing the tow vehicle and keeping everything in control and heading in the direction that the driver needs. If there is any positive transfer, force applied to the truck isn't because the king pin is slightly ahead of the rear axle but because it is much higher than the truck's center of gravity and axle center-line, in the range of 20 inches.