Starting problem

Purchased a used 2002 chevy venture van. It
started with a jump. And ran fine when you’d turn it
off. Purchased a new battery. Same thing, if you
didn’t start the van in 24 house you’d have to jump
it again. Put an altanator and still having the same
trouble. If it sits it won’t start, it’s tring to crank over.
It’s almost like something stays running an drains
out the battery. But can’t find anything running, has
me baffled to what it could be. No noises with the
starter. Please give me suggestions to how I can fix
this. Thanks much