Once you have the 5th wheel turning don't be ashamed to pull forward a short bit to start correcting the turn. This was what made it hard for me. I too had always pulled bumper hitch trailers and would take pride in backing in my driveway with out having to pull forward to correct. My next door neighbor was a retired truck driver and I always thought he was watching me so I tried to show off a bit.
When we brought our first 5th wheel home I made a total mess of the grass on each side of the concrete driveway!! I felt like a rookie. Once I realized how slow a 5th wheel reacts to steering wheel changes I begin to get better. This is why I say don't hesitate to pull forward a time or two for a couple of feet to make corrections. Making corrections while still backing is really difficult with a 5th wheel.