This paper presents a one-dimensional digital twin of a commercial vehicle heating, ventilation, and air-conditioning (HVAC) system, developed in MATLAB and Simulink with Simscape. The model integrates a belt-driven, engine-speed-dependent R134a vapor-compression cycle with a reduced-order cabin thermal network. The refrigeration loop incorporates a positive-displacement compressor, condenser, thermostatic expansion valve, evaporator, and receiver, with the heat exchangers represented using two-phase and moist-air formulations. The cabin model accounts for six surfaces with parallel glazing and opaque thermal paths, solar and long-wave radiation on exterior panels, occupant heat and moisture loads, and speed-dependent external convection, parameterized for validation drive segments. The framework is designed for traceability from component-level state variables to system-level outcomes, enabling staged validation using pressure and temperature signals across the refrigerant loop and cabin temperature response. Validation was performed against controlled climatic chamber data, including a hot-soak period with the air-conditioning off followed by cool-down tests at two steady vehicle speeds. Simulation results were resampled to match measurement timestamps, showing strong agreement with cabin temperature dynamics, with a root mean square error of ~2 °C and correlation of 0.99 in the transient-excluded window. Refrigerant-side pressures and temperatures exhibited segment-dependent accuracy, sufficient for calibration and strategy assessment. Simulation-based screening of cool-down behaviour and refrigerant-loop states provides a reusable framework before repeated vehicle testing, reducing the calibration effort for faster HVAC system evaluation. Overall, the model supports rapid iteration, ensures thermodynamic consistency through enthalpy-based checks, and provides a practical foundation for controller development and future in-the-loop applications.