I have a 1979 ford chateau super wagon. im assuming its an E-250 bc its 3/4 ton.
i have put 2 new batteries in this thing. The alternator is fine. The battery will die if I do not crank it and let it run at least once a week. I looked it over at night to see if any lights were on in the interior and found none that were lit.
The only electronic pawer options on this thing are the door locks and they dont even work right.
How can I tell if the battery is drawing power while the van is off?
It has an aftermarket cd player and the clock on it stays on but that shouldnt be enough to kill the battery.

 

If the alternator has a bad diode it will allow voltage from the battery to leak back to ground. It may not show up on an alternator test.
If you have one battery that is not the same age then it could be pulling voltage down so you would not have sufficient voltage to start the vehicle.

Also aftermarket radios can also leak back form the battery. In addition a bad cell in the battery can allow it to die.
If you have time without use, disconnect the negative terminal from both batteries for the week. Then, reconnect just the primary battery to see if it will start with no problem. If not, then it internal to the battery.