Not being an expert, nor having ridden in the truck, here is a list:
1. Look in the manual and see what the max inflation for the rear tires is. Are they at the max?
2. Has he had the shocks changed from the OEM?
3. Does he carry a lot of weight in the rear of the fifth wheel (can lead to some of the sway as the tail wags the dog)
4. Check the suspension system (on your list) and the alignment of the trailer axles.
5. Is the hitch in the bed of the truck over the rear axle?
6. You might look at adding a towing anti-sway bar to the rear axle.
7. Is the pin box straight? Not bent to one side or the other?
8. Does the fifth wheel hitch have play in it? Does it make a clunking sound starting or stopping?
9. According to what others have said, the front tires need to be a little softer than the rear tires. The Ford manual for my truck indicates 80 psi rear and 75 psi in the front.
10. Without out seeming callous, does your father have any inner ear problems which would magnify the feeling of sway?
Good luck and let us know what you find.
