As an ex truck driver here are a few tips for backing in a 5th:
Always back in from the drivers side. Backing in to the passenger side is called blind backing and you need a spotter and it isn't easy.
When you pull up to back up, pull across the road (from the left to the right) and turn the steering wheel to the left to set the trailer angle towards the path you want to take. (make sure the trailer end is in front of the parking pad before stopping)
When you start to back up turn the steering wheel to the right and watch the back end of the trailer. Go slow
and wait until it starts to turn. Then turn the steering wheel to the left a little at a time until the trailer end stops turning. Now the trailer will turn right or left with just a little bit of steering (around 2 - 4 inches) This will help you control the path the trailer takes without over steer.
You should learn to use your mirrors for backing in and not stick your head out the window because if you stick your head out the window you are blind on the passenger side.
I would suggest using a large parking lot and try to back it in in one of the spaces and pretend it's a RV pad. It will take awhile to get the hang of it but then you will be confident and have less of a chance to do any damage.
Right now I have to back my 60' unit off a narrow road, across my narrow driveway, between 2 palm trees, around another tree and jack knife it in between the garage and another tree. When my friends ask how I got it in there and how am I going to get it back out I just tell them "a helicopter"