Turns out the jolt is a combination of 2 issues: The main problem was the pin box on the fifth wheel trailer. Its a Tilt air rotoflex pin box, which tilts the pin around a horizontal pivot and has a rubber damper to reduce chucking. The rubber pad had torn and slipped back, enough that the pin was rocking and contacting metal when I slowed. This issue was probably compounded by the brake on the trailer being out of adjustment. Awaiting new part.
Seems obvious now, however no one considered looking at the pin box.
Started our journey: 2015 Ford F350 Diesel Dually, 2016 Forest River Vengeance Touring 39R12 toy hauler, Can-Am Spyder RT-Limited.