I'm not speaking from any practical experience sculpting cake, but I do have some practical modelbuilding experience. Mainly trains and spacecraft, but some cars and trucks.
An 18-wheel truck doesn't sound all that daunting, assuming it's a simple box-van. I would start by selecting a round cookie or candy (Oreo, Sandies shortbreads, &c) for the wheels, making eight double-stacks of them, and two by themselves. Then, I'd cut out two pieces of thin plywood, one for the tractor, the other for the trailer, and mount them so that the trailer is high enough to completely clear the cookie-wheels, and the tractor base sits slightly lower, with the front of the trailer overlapping the back of the tractor by at least the diameter of the cookie. Next, I would cover the plywood with a food-safe material.
Then, I would assemble the box-van trailer body from either layers of sheet cake, or possibly from loaves of pound cake or angel food cake, depending on the preference of the recipient, as a simple rectangular block. The cab would go together similarly, except that it would have a more complex shape, and would
I would then build the cab similarly, except that the shape would be a bit more complex. Finally, I would mount the wheels under it: two pairs of double stacks under the back of the trailer, the other two pair under the back of the tractor (with the front of the trailer overlapping the rearmost pair), and the two loose ones set into the front of the tractor.
As long as the thing is structurally sound, and well-supported, it shouldn't make much difference what kind of cake or frosting you use.
As to frosting, there are heat-resistant buttercreams and buttercream-like frostings. I've never used them, but when I was unnecessarily worrying over something with which to write on the side of a pound cake, I certainly looked at plenty of them.