Why does my car lose all electrical power

Why does my mom's Chevy Venture Van lose all electrical power not while driving but when parked and starting it up?

*It has a new battery*
*The Alternator is good*

*Scenario 1: My dad and I had to drive the truck over to my moms van because it wouldn't start and had no electrical power. We parked in the parking space in front of the van, used the jumper cables to jump the van and it turned over and cranked up.

I drove it home, put it in park, and turned it off, then turned it back on, more than once to check its consistency and it starts but not all the time, sometimes it dies down, but it also loses all electrical power sometimes, because when I brought it home and turned it off the door locks work which told me that it had electrical power, whereas as sometimes nothing electrical works and it won't start up, when it is like that the door locks don't even work.

I don't have enough auto mechanic to pin point the problem, just some to do general maintenance. If anyone would be so kind to help me figure out what is wrong with my mom's van it would be greatly appreciated.