As per the instructions here is my very basic car model, which is actually a truck. Navigating around the thing was cumbersome and making sure that the pieces were butted up against each other was another pain. I hope I get better at this over time.
Your truck looks great!
Unity really isn’t designed for 3D modeling, so don’t feel discouraged about how challenging it was! It’s really more about getting a little more familiar with Unity’s interface and tools.