HELP '70 VW camper

Every time I fill up my gas tank, the battery dies. It runs fine for a few days, then when it is time for me to fill up, I fill it up all the way, and then the car won't start. It won't even let me jump the battery. The first time this happened, I charged the battery at home with a charger and took it back to the car and it worked fine. Then, on my next fill up, the same thing occured. What is going on. Any help would be greatly appreciated.

-Jeremy