T** This achievement is glitchy and does not unlock in a timely manner. **
You may have to delete your save data for this to unlock, so you should attempt it either as your first achievement or your last achievement. It is also possible that it just may not unlock at all for some profiles.
In theory, this achievement unlocks after you have successfully parked 15 trailers. In reality, it is a temperamental achievement that may or may not like you. Most people who have successfully unlocked it have repeatedly parked the first trailer 20-30 times on level 21, pausing and restarting as soon as your rig unhooks. Any trailer should work, but that level is reasonably fast to get parked.
Note that this approach generally only works on a clean save, with no levels completed.