The only beeping I've ever run into within our coach was the smoke detector when it's battery needs replacing. If yours occurs only when you switch off the coach batteries, it could be some gas protective device that's sensitive to low or no power. If by chance your smoke detector is dual powered (9 volt onboard plus 12 volt coach), and your 9 v. is shot, that conceivably could be the culprit, but our dual power smoke detectors at home beep when the battery is low in spite of the presence of house voltage too. So this is one you'll have to track down by going inside when you kill the main switches. You should have a CO sensor on the bedroom ceiling and a propane sensor on a galley kick panel, but they shouldn't sound off unless powered and offensive gases are present.
Your auxiliary compressor should only come into play if you parked the coach and leveled it using the automatic feature where the system levels itself and then rechecks every so often, pumping air to any corner that's slumped at all. That compressor is usually just forward of and under the steps. Often though, I am in position to hear dash functions like the radio, etc. power up not long after turning on my mains, but that noise lasts only briefly.
Some may not agree, but I usually don't store the coach without having dumped the airbags. If you leave it in Travel Mode, air in one or more bags normally leaks at least to some degree over time and can result in the rig low in one corner. That's as bad or worse than leaving it on unlevel ground, and the resulting stress at the cap from body twist can slowly crack windshields - been there, dun that. One piece windshields like yours and mine are particularly sensitive. Store the rig on a surface that at least isn't more than a couple inches low in any one corner; reasonably low in the front or back or side to side is okay as long as it's straight along that side - no singularly low corner.
Others here may have better assessments of your undefined noises.
Joel
