...so why would trucks be any different?
The trailer makes quite a difference when backing up. Hang out at your local boat-launch to see examples of amateurs giving it a shot.
The big problem is that the world we live in isn't designed for big trucks. Large vehicles have to break all sorts of traffic rules just to navigate most cities. Accommodating for a trailer often requires making turns from one lane over, or even driving into oncoming traffic first. There are docks that can only be backed into by pulling across two or four or even six lanes of traffic to get lined up. There are stops where trucks have to back up blind, sometimes even into oncoming traffic (and unless the Tesla truck also comes with a Tesla trailer, blind reversals aren't something it will be able to do). Sure, a self driving truck might be good for getting from warehouse A in city X to warehouse B in city Y (a "pin to pin" run) but there's still a huge range of unique traffic challenges these vehicles face every day. Not to mention the art of driving a truck in a fuel and maintenance efficient manner (it involves a lot of prediction). Most people just don't know these issues exist, because they've never driven (or even ridden in) a semi.
I honestly think that Musk is going to find that it's simpler to get to Mars than it is to build a universal self driving truck. At least this could be a good start, though.