Practice, practice, practice and more practice....it becomes second nature.
Forward or backward...One tip is to begin your turn once the object (mailbox) is even with the tires on your trailer. The 5th wheel can be pivotted around the object.
One campground we go to I have to parallel park our 37 foot 5th wheel in a 45 foot long space. RV in front and RV in back of the space. It's fun to watch the reactions of the other RV'ers as I get the trailer into the spot on one try. It took several tries the first couple of times we stayed there. Now it's second nature.