Bit of a can of worms really.
I use the corwoods heater and it's awesome.
It basically needs to be well ventilated. So your safest option is hang it on the back door and use with the door open.
You can though, like me, have it inside the van but either DIY a vent pointing out to the doors, or just work with the door open.
Another option I'm looking at is fitting a spinny vent (not sure what their called) on top of the van to take any gases away aswell.
The way that richard has done it is your best option.
http://youtu.be/AM07hsbkChU