If you want to paint on this trucks. use following templates. You can use them to make and share assets without restrictions.
Tractor: https://drive.google.com/open?id=0B5rGEr-pm_NOYlAyWWtwNDRMVjA
Trailer: https://drive.google.com/open?id=1I_e2mfN-8P7sG2BpvSMzNcuqAP5iJzJx
If you use PNG, the diffuse texture will need to be merged. I recommend using XCF(GIMP). The mesh of this asset is unexpectedly simple. The appropriate combination of normal-map and specular-map greatly contributes to quality. Remember to edit _n and _s when you edit _d.