Hi. sorry for delay.
your mesh object has all vertices separated from each other and vertices affect one on another. This is probably a result of importing a OBJ or DAE created by 3DSMax. You should use an "optimize vertices" option when you import 3DSMax models into ZModeler.
At this point, you can use "Multiple Target" option of "Weld" tool to fix the problem. Switch object to verts level, select all verts and use this tool. a flyout window options are: distance = 0.00; UV tolerance = 1.00; normals angle = 20..60.
when vertices are disjoint, ZModeler draws solid wireframe lines in "Outline" viewport mode. If you see solid lines on unified surface, vertices are disjoint and this is a point to doubt whether model is Ok or not. An image below shows an original model and a result of using Weld/Multiple. You can see a dimmed wireframe on an object - it will give more accurate prelit result; as well as it drops vertices count and mesh/file size of your exported model too.
Using proper material configuration for the trailer allows to render a prelit color texture too (with some minor messing with your default uv mapping). a very draft and low-res shadows render can look like on an attached image.