Hybrid solar desalination systems demand accurate predictive models to maximize efficiency and reliability, yet most existing approaches oversimplify heat transfer in parabolic trough collectors (PTCs) by assuming uniform irradiation and neglecting coupled interactions with solar stills. This work proposes a novel coupled numerical framework that integrates a computational fluid dynamics (CFD) model of the PTC receiver under non-uniform solar flux with detailed energy-balance equations for all solar still components, including basin water, glass covers, and liner. The model was experimentally validated in the arid region of Guemar (El-Oued, Algeria), showing strong agreement with measurements, with a maximum relative error of 5.3 %. Parametric analyses assessed the influence of nanofluid type (Al2O3, CuO, TiO2, SiO2), seasonal variations, and operating conditions. Results reveal that the PTC-solar still hybrid more than doubles freshwater production, achieving efficiency gains above 120 % compared to a conventional still. Among the tested nanofluids, SiO2 delivered the best performance, enhancing productivity by 22.6 % in winter and 16.2 % in summer. Optimal performance was obtained with low nanoparticle fractions (5 parts per thousand), higher Reynolds numbers, and rim angles >= 70 degrees. These findings demonstrate that the proposed coupled CFD-energy balance methodology provides a powerful design and optimization tool for high-performance hybrid desalination systems in arid climates.