Without sounding rude, crude and totally arrogant..........
Idling when the drivers are in the truck sleeping means being either WARM or COOL depending on the season. In the summer you'll bake and in the winter you'll freeze so being comfortable means running the rig.
john has hit it on the head. i have trucks of my own, and when i have been on trips with my dad, depending on the weather, thats why you idle.
NOW, there are alternatives at alot of truck stops like Petro, TravelCenters of America, Flyin J, and a few others, that offer the
IdleAire system.
it offers heat, A/C, phone, internet access, and coax, and the truck stays off.
there are also other systems that offer on board small diesel opperated units, that run off of the trucks fuel, but at a lot less rate of burn than if the truck is running itself.
in todays trucking business, you have to do whatever it takes to save on the fuel.
for car haulers too, having an electric operated hydraulic pump, can also save on fuel, when loading and unloading, to drive the hydraulic ramps on the truck and trailer.