The electrification of transport creates rising and increasingly simultaneous energy demands for large vehicle fleets. Bidirectional charging turns these vehicles into flexible resources that can serve several markets at once. For energy-intensive sites with their own grid level, such as Frankfurt Airport, this creates substantial potential. No transferable concept yet combines spatial charging infrastructure, simultaneous market participation and multi-day planning for a large commercial fleet under real operating conditions. This paper develops such a concept within the ReSkaLa@FRA living lab, using operational data from Frankfurt Airport. It identifies three use cases, spot arbitrage, self-consumption optimisation and balancing services, and unites them in a rolling-horizon optimisation architecture. Its central finding is that the difficulty of multi-use operation lies not in the battery but in four couplings: temporal, physical, spatial and fiscal. A naive formulation overlooks all four. The result is a practice-oriented framework that provides a scientific contribution to V2G research and a directly applicable foundation for integrating large fleets as flexible resources in industrial energy management.