You can't actually export a 3D object as a .truck file. But Undead, what do you want to export from Blender? A n/b or a 3D model? If it's the latter, you need to export it as a .mesh file, with this tool.
Here: https://forum.rigsofrods.org/resources/ogre-command-line-tools.967/
If it's n/b, I...