Bah.. re-train them into knowledge workers while they sit there.. Provide them with a wireless workstation and they can do all kinds of other interesting work for the company while they sit there for hours on end..
Speaking as someone who is sometimes on that road in a vehicle that is much smaller than a truck, I would strongly prefer that they keep staring at that road, and everything else on it.
I'm in such a vehicle too, and I think you're crazy to want that. If they spend 6 hours unnecessarily staring at the road while autopilot is on, they're not going to be anywhere near peak driving condition when they do take over.
Heh, yeah they could definitely learn some other profession while monitoring the truck. But it would probably be against the rules or downright illegal...
Better than staring at the road..