I may have missed something along the way, but if he has a cheap fifth wheel hitch, it's it possible that it doesn't have a pivot allowing tilting side to side which causes the rough ride? I know about chucking and such, but that seems to be more concrete roads and he mentioned asphalt, which makes me think it may be locked side to side that is causing the issue.
Just a different thought...
2016 Jayco Eagle HT 29.5BHDS (ordered 12/30/15, delivered 3/8/16)
2015 F-350 crew cab, short bed, 6.7L PSD, Pullrite Superglide 3300 hitch