A combined theoretical and experimental analysis of the resonant Rayleigh scattering by excitons in a disordered GaAs quantum well with monolayer islands is reported. Experimentally, the coherent resonant Rayleigh scattering is deduced by means of both statistical speckle analysis and passively stabilized spectral interferometry. Theoretically, the effect of disorder is modelled by solving the time-dependent Schrodinger equation for the exciton center-of-mass motion. The disorder potential used in the model includes the detailed monolayer structure of both heterointerfaces, the short-range disorder due to segregation, and a correlation between the upper and lower interfaces. The simulations reproduce all features observed both in frequency- and time-resolved measurements. The comparison between theory and experiment bears strong evidence that the monolayer fluctuations in the two heterointerfaces are highly correlated and have similar length scales.