Could be a couple of different things...
1) You may only have 1 tank, even though you have 2 gauges. The gauge set up is possibly generic, and used in more than one floorpan model.
2) You may have a 2nd tank... and the release valve is located under the rig instead of in the wet bay with the others (if you have one). Look for something like like shown in the photos. On my rig, the 2nd tank handle is just forward of the axles.