Fascinating photos--thanks Doug. Some tanks had a "steam jacketed outlet" (ACF's name, illustration in 1925 CBC) that heated the valve and outlet pipe area. It looks to me like the small trailer had a pump on the far end that sucked the oil out of the dome, through the pump, and into the truck. The heavy (insulated?) hose to the outlet area could be the heating line, with the hose through the dome poked down into the outlet area to suck up the warmed oil. So the other apparatus on the trailer would be water and fuel tanks, and a small boiler. Informed speculation, tear it up Tony ;)
Peace, Rick Aylsworth