luvmydogs,
You asked: "Also, why do you need the motor to be running to get slides in?"
I know on my gasser, with the engine running, the slides get an extra boost of voltage and retract with less strain, especially the heavy main slide. They won't extend if the engine is running, though, but extending doesn't require as much power because the slide doesn't have to be "lifted" first. I'm assuming for some diesels the same would be true.
But I agree, I don't know why folks will idle their rigs for long periods of time. In cold or hot months, perhaps they're doing it for the heat or A/C? I figure it's their rig, let 'em do what they want.
Oh, and I think some guys just love to hear the rumble of their diesels! LOL
~Rick