One of the reasons for creating the system of death camps was that Nazi soldiers and policemen tasked with murdering Jewish people and other undesirables had elevated rates of PTSD. Also, during the Cultural Revolution, the People's liberation Army switched to a lower caliber sidearm because all the executions were giving them carpel tunnel.
You don't want to loose sight of humanity just because you're committing atrocity.
I know that another driving force for the gas chambers was to preserve ammunition.
The earliest versions of gas chambers were essentially "piping truck exhaust into a building." They moved on from that in order to preserve metal (from the piping), fuel, and vehicles for other purposes.
They did it throughout the war and not merely at the start? So weird that there's photographic and video evidence of people being gassed in buildings then.
Edit - Here's evidence that the Germans used chambers (often called "showers") from 1941 onwards:
While I'm sure vans were still used for their mobility/convenience and in cases where not many people were to be executed, Germany created extermination camps specifically to kill people (in more ways than just gassing) and those locations contained constructed buildings meant for execution with gas, not vans.
Even a few concentration camps had their own gas chambers, which were not vans, that were used to execute people that could no longer do forced labour.
When extermination methods were tested to relieve the Einsatzgruppen which had up to that point in the war been shooting people close up, by hand, gas vans were used. As tests. Until Zyklon B was introduced.
I don't know what you're on, but it is a historic fact that before the gas chambers were used, gas vans were a method of murder.