Or, alternatively, do they wait at truck stops on the edge of town and meet the trucks as they come in (much like harbor pilots meet container ships at port entrances)?
Of course if you need to provide all of that to allow for someone to drive it manually around town a lot of those benefits are gone. Who knows, there will probably be some transitional period where all automated trucks are still equipped for manual control but I doubt it'll stay that way for long.
Although thinking about it, the guard could travel in a separate vehicle, and could be responsible for overseeing an entire convoy of trucks at one time.
There’s no reason to build a cab into the truck that is only used for the last mile. Telepresence will do just fine.