Ford uses Nylocks on the spring loaded intercooler boot clamps also. I did say you won't find Nylocks "under" the truck, not throughout the entire truck.
Under most pick ups I have seen, the fifth wheel hitch hardware on the passenger side is directly above the exhaust, often quite close to where the tail pipe is closest to the bed as it humps over the axle. I would remain concerned about using Nylocks in this area.
Otherwise, your points are well stated and well taken about production simplicity and costs driving hardware decisions. Thank you very much for contributing your experience.
As far as Nylocks, one of my best mates, a retired nuclear submarine engineer in the Navy, specified and encouraged the use of Nylocks quite a bit. It's fair to say as in space, there is very little tolerance for loose nuts on a submarine mission.
I still wouldn't use nylocks directly above an exhaust system though.