I can only speak for myself and my experiences.
...this is the largest 5th wheel I've ever owned/pulled.
I was not aware of the possible problems when I bought my unit until I started reading about the pin box/frame failures due to poor design and thin/inferior materials.
In answer to your question...
1) a simple test to see if you have any
problems is to hook up your 5th wheel to the truck and "watch closely" as it lowers onto the hitch. The pin box should not flex as it settles onto the hitch...it should remain in the same plane to the front cap before and as the weight transfers. If it shows significant flex (tilt upwards) do an inspection for tears in metal or weld failure in the whole front section not just where the pin box is attached.
2) quote:"was quite rocky feeling when my wife and I pulled it home
" ...I think the 'rocky' feeling may have been what is called 'chucking'...which is the battle between the truck and trailer as it rides along the road...a jerking-back-and-forth feeling in the cab.
Chucking , Sudden Bumps & Dips in the road... All this puts a Heavy Strain on the whole rig...trailer frame - pin box mount- hitch and truck.
3) I can not say for certain if a... (quote)"might a flexible pin box help mitigate any frame issues
"... but from experience my TailAir Tri-glide Pin box has virtually eliminated the 'chucking' (with the tri-glide rollers) and the Airbag provides a good dampening of the harsh bumps and dips.
However, as I do not have any frame problems at this point (from inspection) I did put out the big bucks for the TrailAir Tir-glide.
No chucking makes it a far nicer ride in the cab and seeing (via the rear view mirror) the air bag doing it's job gives me more confidence that less stress is put on the 5th wheel frame ....lessening... the possibility of metal/weld failure .
See my previous post :Re my follow up
Reinforced King pin box
Hope this helps
'07 WildCat 30 RLBS ~ West Coast Edition ~
Trail Air - Tri Glide Pin Box
2000 F350 DRW 7.3 Diesel Lariat 89k miles
Edge programmer, 4" SS exhaust turbo/back
Vancouver Island, B.C., Canada