my ford van goes dead

I own a ford 1999 e150 van. Recently our van would not start we charged the battery and it worked fine for a few days then dead again. We replaced the battery and it worked fine for 5 days then dead again we jump started it and it worked fine for 1 day then dead again. Could it be the alternator or it there more likely a electric draw on the battery from some source. The ban never dies while driving and the volt meter indicates that the battery is charging while driving. it always goes dead just sitting in driveway.