I have done a couple of couple of 18-wheeler cakes. I start almost all of my sculpted cakes with sheet cakes. Then I can cut them how I want. If you have the time, I would highly recommend Mike MCCarey’s Cakenology Vol 1: Car Cakes video. It is instrumental in making good vehicle cakes.
Here’s a picture of an 18-wheeler I did I long time ago. I was fairly proud of it then, but I look at it now and see lots of room for improvement:
I did one a little more recently, but the child didn’t want the trailer, just bundles. It still shows how you would design it though: