It could be that some of your fuel return lines on the injectors are leaking.
What happens is that if they have some small cracks they will work normally
and not even leak while the van is running.  But when you shut it down at
night it allows air to be sucked in and it drains the fuel back towards the
tank.  So check these small hoses between your injectors and replace them if
they look old at all (these lines are very cheap and can be bought at any part
store).  Also check those two little rubber plugs on the first and last
injector as they go bad and crack sometimes too.
I would almost wager that this is your problem.
Ken Wilford
John 3:16