Congratulations on that deal.
Wait... It needs to be jumped every time you shut it off?
Oh, no! It's worthless, and I'll give you $700 for it.
No, but seriously. That sounds like you got a great deal.
Vans, motorcycles, airplanes, tractors, boats...
They're all the same, only different.
The same common sense works on all of them, so if you know some stuff about bikes, you know some stuff about vans.
If it was mine, I'd check the condition of the battery cables and terminals first.
Clean all that stuff up really good and then see what you get.
Sometimes, a poor connection will cause poor charging, poor starting, or both.
If the battery has been low for a while, it might not be able to come back up, so you might have to replace it.
If you have a good charger, throw that battery on for a while and see if it'll take a charge.
If you don't have a good charger, they are always worth the investment.
If it turns out that the battery is not the issue... Well, it is now, but it may be the symptom rather than the problem.
So if it's not the battery or the cables/connections, then it's the charging system.
Check the alternator wiring, make sure the belt's not slipping, have the alternator checked at a local auto parts store, or swap it with a known good one.
That's all I can think of off-hand.
Again, Congratulations on your new van!