This article considers the identification of state-dependent parameter~(SDP) models for the hydraulically actuated dual-manipulators of a mobile robot used for nuclear decommissioning tasks. A unified framework for calibration, data collection and system identification is developed, and utilized to investigate potential state-dependencies. The latter are associated with nonlinear system dynamics and can cause irregular joint movements when the device is controlled using linear control algorithms. The analysis suggests that a univariate SDP model is suitable for control design. The model has a state-dependent gain, characterized directly from experimental data using a numerically optimized polynomial function of the delayed input variable. In order to demonstrate the practical utility of the SDP model, closed-loop results using a novel non-minimal regulator for joint control are briefly considered.