Depending on how long you're letting it sit for, and what the temperature's like outside, the battery dying could be anywhere from "totally normal" to "parasitic power drain". Whatever mechanics have blown you off on this with "it starts right up" are idiots. As long as it's charging, obviously it's going to start right back up after you drive it over to the shop.
Assuming a drain, you can google how to find it with an ammeter through trial and error. Basically, you check see how much current is flowing out of the battery when everything is supposed to be off, and if you get a significant reading, you start pulling fuses until it drops, then you investigate the system covered by that fuse.
Before that, obvious places to start are: are any lights getting stuck on? This really should be taken care of by the auto-cutoff in the GEM that kicks in after a certain amount of time, but that system could be acting up. Are any relays getting hot to the touch, indicating that they're continually engaging and disengaging? Or, are there any non-stock accessories attached to the battery, like lights or power outlets for the cargo area?
If you're consistently letting the van sit for some time (say, >2 weeks) in between each use, it's might be best to set up a battery tender, and leave it plugged in when you're not using it.