The large-scale integration of plug-in electric vehicles (PEVs) into modern power systems introduces both challenges and opportunities for grid stability. With their bidirectional power transfer capability, Vehicle-to-Grid (V2G) technologies offer significant potential for providing ancillary services, including frequency and voltage regulation. However, the inherent uncertainties from load fluctuations, intermittent renewable generation, and variable EV charging/discharging behaviours demand robust and oscillations free control strategies. The proposed work presents a novel Automatic Voltage Regulator (AVR)-integrated Integral Sliding Mode Control (ISMC) based intelligent V2G control strategy designed to enhance ancillary services support in EV integrated power systems. Unlike conventional sliding mode controllers, the proposed ISMC eliminates steady-state errors and reduces oscillations while maintaining robustness against uncertainties and disturbances. The key novelty lies in the employment of the AVR-ISMC integrated controller in both the active power loop (for frequency regulation) and the reactive power (voltage control) loop (for voltage support), enabling coordinated and decoupled control of real and reactive power from PEV batteries. This dual-loop architecture ensures fast, accurate and robust response under dynamic grid conditions. The proposed control approach is implemented and rigorously tested on a modified IEEE 14-bus system under multiple scenarios, including sudden load variations, random EV fleet availability and renewable energy intermittency. Comparative results demonstrate that the AVR-ISMC integrated controller outperforms conventional methods, achieving superior voltage profile maintenance, tighter frequency regulation and faster convergence with minimal ripples, thereby validating its effectiveness for intelligent V2G-enabled ancillary services support.