Generally the "Green needle" if for the primary air system (the rear brakes) and the Red needle would be the secondary system, but there is no standard/requirement and manufactures can use any color they want. You can easily test this by opening the air drain on the primary and/or secondary air tanks and see how the gauges respond.
Usually there are several (non-brake components) feeding off the air tanks. The coach builders will tap into which ever tank is closest. There would be a "pressure protection valve" (PPV) at the tank where the line feeds from. The valve is supposed to shut when tank pressure drops to set level (like 65 psi) so any leaks in components (like the step slide-out covers, air horn, leveling system, etc.) wont drain the tank completely. But these protection valve will leak internally and allow the tank to drain.
Here's an example of a tank setup (may be correct for your '98) Note the PPV locations