According to the member of the Belzec staff and later commandant of Treblinka, Kurt Franz, a homicidal gas van existed in Belzec prior the operation of the stationary gas chambers. Lorenz Hackenholt is supposed to have been involved in the construction of the gas van, which Franz described as a converted parcel van. Some corroboration can be found in the testimony of Anna Fuchs - the former wife of the gassing device specialist Erich Fuchs -, who stated that her former husband drove a closed parcel van during his Euthanasia time and wrote her from Belzec that he drove a gas van.
"After I got in touch with the Higher SS and Police Leader Globocnik in Lublin, I was sent to Belzec. A camp was erected there. I heard that it was a camp to exterminate the Jews. The camp commenced as extermination camp while I was still there. The killing of Jews was done with gassing vehicles. The vehicles looked like those for post parcels. While I was in Belzec, the gas chambers were built."
(interrogation of 30 December 1959, BArch B 162/5066, p. 150, my translation)
"I remember that there was a Wehrmacht grey, converted parcel van (like a paddy wagon) in Belzec before the gassings started in the gas chambers. The exhaust pipe was directed to the inside of the vehicle to kill with the exhaust. Hackenholt is supposed to have converted the vehicle or had the idea or he was somehow involved. I don't remember how exactly this was, I only have a vague memory of it."(interrogation of 14 September 1961, BArch B 162/3170, p. 183, my translation)
"I rather had the impression that the vehicle had been constructed by Wirth or his men...this gas van was supposedly used only once."
(interrogation of 14 June 1962, Niedersächsisches Hauptstaatsarchiv NDS. 721 Hannover Acc. 97/99 Nr. 10/27, p. 216, my translation)
"I don't know how and when he [Erich Fuchs] came to the Aktion T4. As he explained to me, he was in Brandenburg before he came to Bernburg. He drove around with a car a lot, but also with a closed parcel van, the colour I don't remember anymore. He picked me up a few times with this and drove me from Bernburg back to Berlin. He also drove me to his parents to Magdeburg with it. He often had to repair the vehicle, he told me.
Fuchs stayed in Belzec for seven weeks. I wrote him and he replied with the address SS Sonderkommando Belzec. He explained to me in detail that and how he installed the gassing devices in Belzec and later also in the other camps Sobibor and Treblinka. He said that at first he drove a gassing van, the exhaust fumes were lead into it. But it took too much time. The Jews also rioted in the vehicle."
(interrogation of 28 March 1966, NIOD 804/47, p. 254 & 257, called to my attention by Jonathan Harrison, my translation)