The gas vans were designed to kill Jews on the run. With carbon monoxide pumping in the back, the Jews would more than likely experience confusion, dizziness, tightening of the chest, and many other symptoms, all eventually leading to the victim being knocked unconscious. Once in this state the victim would then die in their sleep.
The Nazis would load the vans with 70-90 Jews each. They would be stripped of their clothes before hand. It would take approximately 6 minutes for the group to die.
The gas van's exhaust pipes would be routed to the inside of the truck where the Jews were kept. The carbon monoxide would kill the Jews then.
The Inside of a Gas Van
There would be 70-90 people cramped in here only to die on top of each other.