Years ago when I first attempted to back our new 5th wheel, I met with minimal success. Could not understand that as I could back our 20 ft. cargo trailer on a dime.
Anyway...I laid some yellow vinyl tape behind my 5th in the direction I wanted the wheels to track. Backed up a few times watching how the rig reacted as I followed the tape. An hour later it was an easy task.
Just another thought!
Best Wishes!