[mundy5]

I would consider 3 options.
1. you have a loose or corroded connection from the battery terminal
or
2. your alternator is not charging the battery properly.
or
3. there is a parasitic drain somewhere (I've read others with a broken tail light bulb have had battery drains until they replaced the broken bulb).

you can get your alternator checked while it is still in the van but it's not as good as bench testing it which requires you to take it out of the van.

I've had both tests done in auto parts stores like o'reillys. autozone does it as well for free.