Film explanation...I think. Maybe. (spoilers)
This isn't a defense for this film. I didn't hate it, but it wasn't exactly the worst piece of crap I'd ever seen. I'd rate it at like a 3/10, which is pretty bad, but watchable if you can turn your brain off. Meaning...I was mildly drunk when I decided to watch it, and fairly drunk by the time I finished it.
What I gathered, is that this truck was actually a traveling gateway to and from hell. The hood ornament and constant flashes of the three-headed dog shows it to be Cerberus, the guardian (there to keep people from escaping mostly) of the gates of hell. I think it might have been a reference back to Greek mythology, when Hades was an actual physical place that could be entered.
The front trailer, was the "engine" for this hell gateway truck, it was a processing factory for the souls and bodies of victims to keep itself running.
The back trailer was the actual entrance to hell. I don't believe the Cerberus was actually IN the truck, but that you could sense and hear him from 'our' world, by means of the gateway in the back trailer.
The truck itself, entices people to it with the intention of most likely possessing an individual, so that it can use that individual as an operator. It may run on human parts and souls, but it might require an individual to seek out the actual victims...or there wouldn't be someone to load the bodies into the first trailer.
Finally, and this is just total assumption with no real basis to back it up, the reason behind the crazy guy in a 70's casual suit. When he was chasing down the blond haired guy later on, he was shouting "what'd you do with my truck??", meaning he wasn't a victim trying to "save" people, that he was indeed possessed and attempting to kill people to feed to the truck. When he caught up with the blond one, either he lapsed back into momentary sanity, or the blond one got the advantage...70's suit man was killed. Perhaps his 'possession' naturally jumps to the next person, bringing with it a part of his personality at first, along with his insanity. The blond one took on his identity (clothes and all) and went directly back to 'his' truck with the intention of killing everyone. He gained a coherent sane moment for just long enough to allow the girls to render him unconscious.
So in summary, tl;dr, Truck is a traveling gateway to hell, running into other vehicles on the road to either kill or possess them for fuel. Truck has no point other than it is an entry point to hell, ala greek mythology. 3-headed dog was just there to hammer home the point of it being a hell truck.
Also I have no evidence to back up any of this, I was just bored.