WARNING: This page is about a different car, the 2015 RAM 3500 HD and 2015 RAM 2500 HD. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Cylinder Tank Valve, Connectors & Lines
Courtesy of CHRYSLER GROUP, LLC
A 1/4 turn, manual (master) (2) and the cylinder tanks shut-off valves (1, 4) are located in the cylinder tank cover access hatch.