The more you can keep the end (tailgate area) of the truck under the nose of the 5th wheel the more you will "turn" as you back up. Once the "end" of the truck comes out from under the nose the more you are moving the front end over and the less you are "backing" up. There have been a few times where I have gotten the 5th wheel lined up where I wanted to back it into but the truck was still at an angle, so I just dropped the landing gear pulled my truck out , straightened out the truck and hooked the trailer back up then backed it in.
Good luck