basement air pressure gauges in 2000 lxi

[luckyron1]

I'm in the process of setting up the replacement of my ailing 110 V
auxilliary air compressor and re-discovered 2 pressure gauges high up
in the second storage comparment on the drivers side. These are just
around the corner from where the aux air compressor is mounted to the
floor. I figured out which one serves the aux air compressor and I
note that the pressure reads 17 on the outside white ring on the gauge;
and I also note that you can adjust the psi when the aux air compressor
is running. My question on it is: what psi should it be set at?

As for the other gauge, it reads 0 psi right now and this is while
we're hooked up to shore power and neither the aux air compressor nor
the 12 v toilet compressor are running. At the same time, I note that
my engine compressor gauge (on the dash) reads 120 psi. What does this
other gauge serve and what should it be adjusted to??

Thanks guys!

Ron & Ruth about to head for Galveston in 2ooo lxi.