Trying to follow the progression.
Within 30 minites " it " happend again. What is " it " and when did " it " happen before ?
Why are you jumping the battery,,if the battery is new ?
And how does jumping the battery help ?,
How long have you been jumping the battery ?
Do the volts go down to nothing when you first turn the key and try and start the van ?,,,
,Or do the volts go down to nothing because you have tried to start the the van over and over and the van will not start ?
If you brought it to a guy that does electrical work and he couldn't find anything,,
Maybe its time to bring it to the chevy dealer or a shop that repairs chevy vans in your area to see if they can't find any thing.
When the van does start,,how is the vans performance,,idle,,acceleration,,fuel mileage ?