Most newer trucks use about .5 -.7 gallons per hour on long idle. If an APU uses ~half of that - do the math and tell me how many hours you would have to idle to make an APU pay for itself. And then convert that to miles driven per year, to see how many years it would take. Because from what I have heard, APU's generally use between .25 - .33 gallons per hour.
That is only the beginning point. You can cut a lot of wear and tear on the main engine. You can also stretch out oil changes due to less soot from in the oil from running the truck while not running down the road and up to temperature.