A monitor probably needs more knowledge than a driver (need to know the specifics of the product, how to fix problems etc.), and has the same downsides (long hours, not especially exciting job, bad work hours, sitting all the time etc.). If anything I would think they would be paid more.
In my view the monitor would be a driver with standard knowledge and experience who could drive if the autonomous system encountered an edge case or for last-mile, i.e., from the freeway to the shipping or receiving facility.
Under those circumstances a load traveling ten hours with six hours of loading and unloading time would require two hours of active work and fourteen hours of standby.
Any driver would accept a lower rate for this kind of arrangement.
It’s not the same process as the autonomous urban driving being attempted by Waymo et al. Driving from Phoenix to Dallas on the highway is far simpler than driving around Phoenix.