Are you sure that's a fifth wheel king pin connection, to a hitch with jaws that enclose the king pin???
I have a suspicion that what you're calling a king pin is actually a farm/utility/atv trailer pull-pin hitch, which is entirely different than a fifth wheel king pin hitch...
Also, the spirit and purpose of most of these state double towing laws requiring a fifth wheel king pin hitch on the first trailer, is for the hitching location to be over the rear axle of the truck doing the towing, not out behind the rear bumper.
The purpose is to ensure that the first trailer doesn't have a tendency to sway about, on a bumper location hitch, well behind the truck's axle, while forces are acting on the rear of the first trailer, by the last trailer. With the first trailer on a bumper hitch location, all that swaying going on behind the truck is transferred to the truck.
The forces the first trailer is exerting on the truck, are amplified by the hitch's location far behind the truck's rear axle. This can cause the truck to go out of control.
A swaying trailer has much less control over the truck when the hitching location is over the truck's rear axle, rather than 5 to 10 feet behind it, on the bumper. That's a long lever arm for the trailer to use to swing the truck about.