possibly a deadly situation with a corroded battery terminal

Does your positive battery terminal have 2 wires coming off of it - a thick one going to the starter and a smaller one that gives battery voltage to power the rest of the truck? If you have corrosion at the point those two wires meet then I could see how you might lose battery voltage to the whole truck even though it was running and the alternator was working. I don't know what you did to fix the problem but you might want to take a close look at your battery cables and replace them if corroded.