Near where I live the majority of Tesla supercharging stations are connected to fuel cell < - > natural gas gizmos, so in fact what you're describing is already

Near where I live the majority of Tesla supercharging stations are connected to fuel cell < - > natural gas gizmos, so in fact what you're describing is already done. Commercial power billing is typically for the peak draw of an installation over a period of time, so there's huge incentive to chop the tops off of these peaks with fuel cells and batteries. The DC Fast charge power requirements are enormous and I'd be surprised if more than a small fraction of gas stations had direct grid ties suitable for reasonable DC Fast charging.