Thats a ground issue or posative cable issue, If the altenater went out it would still run the battery down slowly untill the truck dies but the headlights would slowly dim not just quit. If its loosing instantly its a probally a ground, a shoted wire large enough to make the car loose all power would blow the main power block fuse. I would trace ground from battery alll the way to body ground and check the ground from motor to body. If and when these are clean start checking the posative from battery to starter and then your main to the fuse block for good connections.
