This paper develops and validates a deep reinforcement learning-based decision support AI agent to optimize patient-hospital disposition decisions during a severe multi-casualty incident (MCI). The AI agent optimizes patient transport decisions by considering patient acuity, specialized care needs, hospital capacity, and transport logistics. We integrated the AI agent into a web-based command dashboard called Master and conducted a user study with 30 participants (6 trauma surgeons and 24 non-experts) to evaluate three interaction modes (human-only, human-AI collaboration, and AI-only). We demonstrate that increasing AI intervention improves decision quality and consistency in 20- and 60-patient MCI scenarios in the Toronto area. The AI agent outperforms trauma surgeons (p < 0.001), demonstrating that non-experts can achieve expert-level performance with AI assistance (without assistance, performance is significantly reduced, p < 0.001).