Truck should have dual field alternator and electronic regulator. I am not positive on a 72 but as said if the gauge on the early ones go you can get a no charge situation. Pull the cluster back or poke your head under there and take a look at the gauge. Confirm power to all connections with meter and look for any melted wires in case something was not noticed on initial inspection of system. If you have another regulator laying around give it a whirl.