NO......
Flex/movement in pin box can occur with traditional 5th wheel hitch
When they had the overhang cover off did they hook and unhook 5th wheel to see WHERE the movement was coming from??????
A little 'flex' is normal (1/4") but 1" is NOT normal.....something is moving
Might not be pin box structure/support/cross frames.
But could easily be sidewall(s)
Have seen where sidewall bolts/mounting plates were broken and sidewall lifted when pin box was 'loaded' BEFORE the horizontal frame member actually moved.
Lag bolts had to be installed in a 'W' pattern to re-attach sidewall to horizontal frame.
1" movement...something is broke.
Find someone with a 5th wheel hitch and hookup for a test.
You will have same movement.