The vent is not for gasses, but for warm air. The system works due to air entering at the bottom back of the fridge, then passing over the "fins or heat exchanger", warming, and exiting via the roof vent.
Your vent plenum only needs to stretch from the top of the cooling fins to the vent through the ceiling. The sides of the fridge need to be sealed to ensure the airflow is uniform in movement from beneath the fins to the roof.
Craig and Carol
1997 34' Excella 1000
1970 27' Overlander, International
2009 Ford F150 5.4L
ProPride hitch with 1400# bars
Past President Southeastern Camping Unit (12)