It all depends on how much else he was carrying. Vision wheels and tires can make his rear axle be the limiting factor since they can easily carry 4500 each. Flat towing a toad adds almost zero payload. Finally, we have no idea how it was optioned, so no way to guess an accurate weight. Since he has Vision 19.5s, my bet he knows the math.
It could be a 9.5 Rainier triple slide. I put a reasonable dry weight at 4200. Sure, it won't stop as fast as my F450 not that most DRWs can, but as fast or faster than most DRWs carrying 11.5 TCs. Because it is newer technology and a Ford.
When I had my SRW, I simply watched how much we carried. Way under the axle weight and under the tire limit with 19.5s.