2001 E350 battery drain

First off, I will admit that I am not much of a gearhead, but any insight would be helpful. I have a 2001 Ford E30 Passenger van that seems to have an issue with a power drain. it i good shape and has only 130 000 kms on it. I have replaced the battery twice no in the past year. It starts fine with a boost, but if it sits for more than a a day or 2 it dies completely. However, i have also left it for over a week before and it started right up. No shop seems to be able to detect some sort of drain on the battery while the van is turned off, but they all say this must be the problem. any ideas? thanks.