We're in the process of buying a pickup truck and truck camper for off-the-normal-path explorations. I understand that it's common to have the rig set up so that the truck engine/alternator will keep ...
There is nothing for the alternator to 'know'. When the batteries (any of them) reach the alternator charging voltage (~14 - 14.2), charging stops. In other words, the 'feedback' is the battery voltage.