I’ll leave the scanner question to others here much more familiar with their use.
To add to Fred’s succinct tank explanation, the rear (red needle) tank is often called the primary, probably due to it providing rear air brake pressure, a safety concern of more importance than the horn, door seal, air bags, or step cover, etc. There is a one-way valve between the two tanks so when the secondary fills, air can go into the primary but the brakes’ supply can’t lose air by way of it leaching back into the secondary (green) tank. When pressure reaches a low point, the engine compressor/air-dryer kicks in to boost back toward 120-130 psi. When it gets there, you hear the moist air release of the dryer purge typical of diesel trucks and motorhomes.
Note that when you park and level the coach using auto-level, typically in a campsite, the HWH system checks for level every so often and uses a small auxiliary compressor to readjust if needed. Just a person moving from one part of the coach to another can affect level enough to trigger that, but usually the pump comes on only occasionally as an airbag seeps a bit of air and that corner needs a boost. The aux pump on yours is under the curbside front between the genset and steps. The OEM one isn’t as resistant to road debris and wear as we’d all like, so many here have replaced theirs with ViAir or other quality compressors. The air line, solenoid, and wiring to it also can fail so it won’t help keep you level. Be aware the aux pump is only an auto-level component; it provides no benefit if you level manually.
Reckon I got off topic a bit, but many new owners aren’t aware of the “other” compressor.
Joel