How exactly does an APU work?

Yes, has it's own heating and AC for the cab.
Yes, most keep the coolant warm. Thereby keeping the engine warm.
Yes, they will automatically charge the main batteries.
Yes, it provides power to the batteries. Anything run off of them will stay running.
There are many levels of noise that they generate. Some are very quite, some shake the whole bloody truck. Fuel fed off the main tanks. Can be expensive to fix and hard to find parts for the more "off the grid" brands. Use about 1/10th the fuel that the main engine uses per hour.

 

All the above depends on the brand and how it is set-up...
There are even battery based APU's that only do heat/cooling for the driver...thses may/maynot use a pre-heated/pre-cooled mass to store heat/cool for later venting to the truck.
Just like bunk heaters there are wide variation from brand to brand...
Many use a small diseal motor, with an alternator to make DC, then have an inverter to make AC. Some may/maynot be tied into thectrucks cooling system...advantages both ways. Same for combined or seperate heating/cooling.

 

IMO, ThermoKing has the best APU. Since they deal with reefers too, there is a large cross country network of shops if you ever need repairs or maintainance. The one I had last was 5 years old and had only one problem with the AC condensor which broke when Blue Beacon broke the housing and punched it into the blades. One day I accidently over filled the oil so I drained some out. Out come a thick sludge from lack of maintainace from our shop. I watch them change the oil now. Of course I had the belts changed a couple times. It's like a Timex, it keeps on running and running.

 

You are looking around ten grand or a little more. But with the fuel savings, one will pay for itself in a year and a half.

You could use 18-24 month financing and the fuel savings will make the payment. After that it's pure savings and less wear and tear on your truck engine.

If you change trucks, you can swap it over.

 

It's a package deal, the housing box itself bolts to the frame.

Here's a link to their website with all the info you need.

http://www.thermoking.com/products/northamerica/auxil/main.asp

 

I purchased a used one on line and it cost me $ 3000.00 installed .It works like brand new one. Comfort Master 3 cyl isuzu diesel engine .I love it.