This question has been asked a fair few times, so I'll make a dedicated topic about it showing the location of the templates for each of the trailers. I'll use one as an example, then you simply just adapt the name of a particular trailer for which you want the template for.

First up, the location of the DEF files. You can find them here:
Note: Change the X to wherever you have installed the game. By default, this will be your C drive, but if you have it elsewhere, make sure to adapt. This is where workshop mods for a particular game get installed as well.

Inside this folder, you'll see all of Fae's trailers in this particular trailer pack. We're going to look at the Utility 3000R for it's template and DEF files. But if you want to do the Hyundai Reefer, you'll go to the faelandaea.htl_rfr instead. Now, for the Utility 3000R, you'll go here for the skin DEF files:

In here, you'll be looking for your_company.sii and your_company_settings.sui. Copy these two files to wherever you've made your base mod for skinning this trailer. For example:
Note: I'm using mymod as the base folder in this example.

Make your necessary changes to the SII and SUI files as you see fit. Also, rename these SII and SUI files to something like myskin.sii and myskin.sui, as this will be used as a unique identifier within the SII file itself.

Next up, is the actual UV template for the trailer. For the 3000R, you can find those here:
Note: Like above with the DEF file, replace X with where your game is installed.

As an aside, in that paintjob folder, you will find all the trailers that Fae has put in. Simply look for the trailer you're looking for if you plan to skin something other than the 3000R.

Now, you may have noticed there is a dedicated folder called your_company, this is the actual location of the UV template itself, which is a simple DDS file. You'll need a program that can handle them like Photoshop with the DDS plugin, or Paint.NET, or GIMP or something. Whichever is your favorite editor. Make a copy of the DDS file and place it inside of your mod like so:

You're free to change the name how you want, so long as your DEF files match. I recommend to always use lowercase because for those not aware, Windows is NOT case sensitive, unlike those on Linux where it is. (this is thanks to a minor bug I found when testing the trailers once before for something lol)

One last thing of note. The TOBJ file will need to be modified and it is not your ordinary text file. You will need to either know how to hex edit the file, or use another program to do so. You can use Mod Studio if you wish to edit the TOBJ, it does come with an editor for that. Me personally, I've found success with Elitesquad Modz TOBJEditor[forum.scssoft.com] and it has worked for me. But use whichever is comfortable, point is being able to edit the TOBJ properly to point to the correct DDS file for your skin.

I hope this helps as this has been a somewhat common question I've seen between here, the forum topic, and in Discord. As always, feel free to ask any questions and one of us here will be able to answer, or forward them for Fae to answer (or relay, because he is a truck driver of course lol) and all that.