Hello!
It's good to take a look at the system in operation - youtube has the movies. There's even a competition about who can unload his(her?) transfer truck faster. To me this system seems to be a waste of engineering and space - I can't get over how long the trailer tow bar is in the photo above. Moving the trailer box onto the dump truck seems very complicated, too - an operation where so many things can go wrong...
But if you want to model it, take a look at the Italeri truck models. Those give you nice, long frames, that maybe wouldn't need stretching. Got to watch them closely, though, because some of their trucks are totally Sci-Fi under the hood. They also have two axle trailers in their line - good base for the trailer you need. Check out those:
This one is quite nice:
This could give you the box sections:
And the trailer:
Hope it helps - and be sure to be posting pictures here! Have a nice day
Paweł