Adding a trailer to ETS2 Studio is not easy at all. This feature is actually intended for experienced modders who know how trailers are created and who know their way around Blender. For the moment, trailer templates are compatible with 50keda's Blender2SCS plugin. In the future, we'll test how things work with SCS Software's Blender Tools as well.
If you take a look at the folder structure of the application, you'll see that we have a couple of additional folders:
Default folder is meant to hold trailer templates that we provide with the application. Templates folder is meant to hold templates that users add to the application. Includes folder holds the files that are used to include the trailers and traffic (AI) trailers to the game.
If you take a look at the structure of a template, you'll notice that it's now structured like the exported trailer mod would be structured. It has the same file and folder structure, with the exception of some files and folders containing variables in their names.
Application replaces these placeholders with proper values when exporting mod.
Also, the same variables are found in the sii files that are now a part of the template. Before, the application was creating these files at export, but now it just takes them from the template. This way, everyone can tune their templates however they wish.
One of the most important elements of the template is the Search String. This is the part of the internal paths in PMD files, TOBJ files (...) that needs to be replaced with the new trailer Internal Name. Usually, the Search String matches the folder name where you originally exported the trailer with Blender. If unsure, you can always open some of the MAT files with a text editor and check the folder name in them.
One of the material files in a trailer template mod has a line like this:
texture : "/vehicle/trailer_eu/coolln/textures/cool_liner.tobj"
Notice the coolln in the path. That is the Search String.
Of course, all PMD files should be exported from blender to the same folder, together with all other necessary files (materials, textures, etc...).
I think that's basically it. Take a look, try it out, and let me know what you think. :)
And, of course - if you need more info - just ask.