This paper presents a RAPS, namely the regenerative auxiliary power system, for the vehicle with special accessory power systems. Taking city buses and delivery trucks as examples, service vehicles keep engines idling to power their auxiliary devices (e.g., refrigeration systems and hydraulic pumps). The potential fuel savings brought on by the electrification of these auxiliary systems are first quantitatively analyzed over a typical drive cycle for a delivery truck. The RAPS is then designed, and its components are sized in accordance with the objectives of compactness and cost-effectiveness. By introducing the proposed RAPS into a conventional delivery truck with an internal combustion engine, the powertrain can be treated as a hybrid because of adding an extra battery. As a result, to pursue a high overall efficiency, a holistic controller is presented for determining how and when to recharge the battery while minimizing the auxiliary system's power consumption. More importantly, the proposed RAPS saves about 7% fuel when compared with consumption by conventional service vehicles.