It pays to check the obvious. Like most VW's, the radio stays on when the key is long gone. My kids like to play in the van and they often leave the radio on. A couple of days go by and I've got a dead battery on my hands. Break out the charger again...

Another possibility is that you have a crappy battery. Here is sunny SoCal, my batteries frequently die an earlier death than I used to experience when we lived in Alaska. The heat and frquent, short driving hops just kills 'em. See if you have a dead cell(s). Charge it up and then, after it sits an hour or so, check the cells with a battery electrolyte tester.

Good Luck.

Tom Graves
'82 Vanagon L
Pasadena, CA