Alternator

First time this had happened is when my girl friend was driving my truck home from work. She said she started it and as it ran the lights started to get really dim outside and inside. Then she said my gauges all went down and eventually the truck died. She got it jumped and drove and it died on her again. She got it jumped three times before she got it home. I figured it was the alternator so I replaced it. I put a good battery in there (not new) and new alternator and drove for about 1,000 miles. And now its doing the same thing as before. What is the problem?