1. A= additional or alternative or anal... or something. So an APU is some sort of "power unit." Most of us call it simply a generator. That's my guess...
2. If you have a BIRD system like I do, then there have been numerous threads/messages here on the Forum explaining how they work. Since it has been awhile, I will guess that the generator first charges your coach batteries, then once those are charged, the BIRD system allows charging of the chassis batteries. Doesn't it work just the opposite from the alternator? I don't know...
3. I have not a guess.
4. My coach is a power hog. If it's not plugged in or with generator running, then the coach batteries will deplete very quickly. Many times I will get parked and before I can get the slides out and leveled, it's generator has auto-started. The slides and hydraulic jacks seem to draw a lot of power. I suppose that is also with a few other things drawing power, like TVs, DISH receivers, insta-hot, fridge w/ice maker, computers, cell phones charging, etc., plus the regular stuff that always draws power like the engine and transmission computers, etc.
I purchased a Northern Power 3000w portable generator that I keep in the pickup. It has a 75' remote start so I can start and stop it from anywhere inside the coach. Most of the time now if we don't have shore power, then I go start the portable generator before I put out the slides and level. It's just a routine I have grown accustomed to - power first, then slides out and level.