I don't see where the vent is missing, just installed incorrectly.
From the link above:
"Thor California is recalling 64,000 MY 1997-2006 Tahoe, Wanderer, Jazz, Vortex and Mirage travel and fifth wheel trailers equipped with Suburban and Atwood furnaces. The furnace exhaust vent was improperly installed. The furnace exhaust vent will allow leakage of fumes which contain combustion products, including a high concentration of potentially lethal carbon monoxide gas. Fumes could enter into the trailer and could result in serious injury or death. Owners will receive instructions on inspecting the furnace vent. If an owner believes the vent is installed incorrectly, is not sure how to inspect the furnace exhaust vent, or needs assistance in getting the furnace exhaust vent installation
inspected and corrected, owners should contact Thor immediately at 1-877-870-2638 or via the internet at http://www.thorca.com. The recall is expected to begin on June 30, 2006. 06V-208"