To realize the full potential of simulation-based evaluation and validation of autonomous ground vehicle systems, the next generation of modeling and simulation (M&S) solutions must provide real-time closed-loop environments that feature the latest physics-based modeling approaches and simulation solvers. Realtime capabilities enable seamless integration of human-in/on-the-loop training and hardware-in-the-loop evaluation and validation studies. Using an open modular architecture to close the loop between the physics-based solvers and autonomy stack components allows for full simulation of unmanned ground vehicles (UGVs) for comprehensive development, training, and testing of artificial intelligence vehicle- based agents and their human team members. This paper presents an introduction to a Proof of Concept for such a UGV M&S solution for severe terrain environments with a discussion of simulation results and future research directions. This conceptual approach features: 1) richly detailed severe terrain environments, 2) vehicle systems with multi-body dynamics, 3) Terramechanics-based tire-terrain interactions, 4) physics-based exteroceptive sensor models, 5) modular ROS autonomy components, 6) vehicle energy management and electric motor models, and 7) a user configurable dashboard for co-simulation coordination and model parameterization for automated M&S testing.