So what's that thing on the back, you ask? Why, it's another kind of rack, a "Hippo" (used for water or fuel delivery.)
As always, we start with a CAD
This one has no electric stuff nor much in the way of moving parts. Frame is plastic tube, and bottom, and box are just styrene sheet. The tank was trickier to make; it's printed, but it's a weird shape (ie irregular, not just a squashed cylinder.)
Also 1:10 scale means it's big, so had to be printed in chunks even on my big printer. To give the illusion of the rings on the tank, just printed short chunks scaled slightly larger; that's the grey glued onto the blue.
More cat scans, and then it starts to come together
And some greebling and paint later
Has the same rollers and underside frame as the flatrack and the drop-side boxrack, so they're interchangeable on the truck or trailer alike.