Actually the tillerman does more than just being rear driver , they help get equipment of the rigs and they man the pumps on the truck along with controlling the aerial ladder for the interior crews that go onto the roof or need access through a window. They're basically an engineer.

It doesn't really take any special training , all they're taught is how to navigate through a maze in various ways then they have to reverse drive to be able to back the truck up out of a spot along with backing into the station. It's not that difficult to operate a tiller , all you do is steer in the opposite direction of the way the front cab is going then as the rear swings around you straighten the wheels out , the only difference is that the driving is done at speed and fast reflexes are important when operating those types of trucks cause one bad move and someone will get hurt and you would also damage the truck too.