A mobile hydraulic cooling assembly has a hydraulic fluid reservoir with a second interior surface offset from a first interior surface. The portion of the second surface offset is not opposite any of the first interior surface in the vertical direction. An air chamber, into which air passes into or out of through a heat exchanger, has an angled exterior surface formed from the reservoir which delimits the air chamber. A fill entry closure which covers a fill entry into the reservoir is accessible by the hands of an operator when a moveable closure of the assembly is in an open position. At least two reservoir portions delimiting a hollow of the reservoir overlap a bottom facing surface and bottom of a truck rail when the cooling assembly is mounted to the truck rail.