Its your alternator! Just to double check, get the battery fully charged and checked. put it back in the truck and start it, with everything on, your volt reading at the batttery should be about 13 volts. Let the truck run a while and watch the voltage, if it steadily drops below 12 volts @ idle with load, then you alternator is toast. Get one from Napa or auto zone, NOT from advance, unless you want to replace it agian soon.