An interesting point.With the exception of the van used at Smolensk and Minsk in July 1942, all vans were used against Russians and Belorussians, small numbers of Jewish forced labourers at the Mogilev camp were killed in them, but they played almost no role in the killing of at least 30,000 Jews in the military zone during 1942. All other documented actions involved shootings. The vans were not so easily deployable to off-the-main-road ghettos.
The development of the mobile gas-chamber, using either bottled CO or the vehicle's own exhaust as the killing agent, is usually presented as a stage in the unfolding of a program of extermination of the Jews.
The common explanation given is that they were invented for the purpose of killing Jewish women and children because the German Police units did not like shooting them.
I myself have thought that the reason for their development was not related to any anti-Jewish camoaign, but rather to the extension of the "euthanasia" of the inmates of mental hospitals in Poland and then the Soviet Union. The use of CO, whether bottled or drawn from vehicle exhaust, would have been modeled on the practice of the T-4 program in Germany.
As I see it, it was only after gas vans were used to empty mental hospitals in occupied Soviet territory (with the assistance of the hospital staff in some cases) that they were then diverted to actions against Jews held in various ghettos, the ongoing action at Chelmno being the prime example.
I find it hard to understand why a gas-van would be sent to Auschwitz of all places. That would be a bit like shipping ice to Antarctica. So far as I know, all other accounts of the use of gas-vans are related to locations where there was no stationary killing installation, eg at Semlin in Yugoslavia.
It may be that in September 1944, Police vehicle 71642 previously assigned to EG B passed through Auschwitz on its way west in the context of the general German retreat in the face of the Soviet advance. As of that date the Soviet Army was just outside Warsaw.
In that regard, the testimony of Goiny-Grabowski at the Hoess Trial is the most reliable, to the extent that he says that a German Police with a number of vehicles came to Auschwitz in the course of its retreat from the advancing Soviet Army, which was approaching Auschwitz. That unit may have had two gas-vans among its vehicles, or that detail may have been invented; the item about the vehicles in question bearing an image showing a human head pinching its nose with one hand sounds particularly far-fetched.
Whatever the case may be, all gas-vans deployed on the Eastern Front must have been withdrawn to the West during or before the general German retreat, since nonewas ever captured by the Soviet Army or by partisans. After their withdrawal, they must have been dismantled since none was ever found in Germany either.
It seems extremely unlikely that the camp staff at Auschwitz or any of the surrounding camps resorted to using the killing capacity of a gas-van when they had a large killing capacity at their disposal on site.