Maybe you haven't thought much about them, but I know you've seen a reefer unit. It's a regular shipping trailer with added insulation and a unit on the front that controls heat and a/c inside the trailer. It runs on diesel, usually from it's own fuel tank that's attached to the underside of the trailer. The reefer is programmed for a certain temp and the unit pumps heat or a/c into the trailer as needed to maintain that temp. They're used mostly in the food transport segment of trucking - whether hauling frozen meat/veggies/ice cream, you name it - but they're also contracted to haul things you likely wouldn't expect, like paint. Paint that's been frozen is inferior and unusable, so the reefer is used to keep the trailer temp above freezing.
While getting our generator repaired at a ThermoKing dealer, I spied the largest reefer unit I've ever seen:
I'm six feet tall, and even after subtracting the six inch platform this beast is displayed on, it's still well over a foot taller than I am.
I've gotta wonder, just what are they shipping that needs this much oomph?