If you want to use a 3 way fridge it really needs the rear to be against an external wall , never seen one vented any other way.The fridge enclosure should be sealed from the living accommodation, so that the burners flue cannot leak into the living accommodation. The sealed part should also have a gas drop out (hole in the floor) under the burner.
Worst case if the burners flue does not vent properly then everyone in the van dies from carbon monoxide!!! , is it worth the risk ?
12V Compressor fridges are expensive , but what price on life ?
or cant you swap the fridge location with the water carriers ?