Blender and SCS BT TOOL are the best tool for ETS2 modding.

Right now it have new Blender 2.80 and it is matter of time SCS to put out they SCS BT Tool for that version of Blender ... for that i just offert you to wait the new BT Tool then to start modding :)

that is SCS MOD page
https://modding.scssoft.com/wiki/Documentation/Tools/SCS_Blender_Tools/Download

Now about your question :)
if you have ready 3d model ... just you need to make .sii files
if you dont then make 3d model then same need to make .sii files :)

cargo and trailer are separate now in ETS2 and in general they are combinate by .sii files witch make more easy to combiante differnt trailers and cargos without need of 3d editing

Any cargo had .sii files into:
def\cargo\ - here is add the the Cargo name, mass and etc.
def\vehicle\trailer\ - here is what is type of tailer and Cargo like 3d object

more easy for you will be ... just to look into SCS extarct files of the game and follow the cargo/trailer files to get the idea :)


Easy way for work is:
copy all def trailer files "def\vehicle\trailer\ ......"
and edit the .sii files for your need then make the cargo .sii file


FOR EXAMPLE :
lets take one cargo boiler_parts

Cargo like JOB work
def\cargo.dlc_oversize_cargo.sii --- it is file where you add the cargo in game list
def\cargo\boiler_parts.dlc_oversize_cargo.sii --- cargo description
def\cargo\boiler_parts\goldhofer.mpa_k.boiler_parts.dlc_oversize_cargo.sii --- cargo description

3d Cargo and trailer
trailer_storage.dlc_oversize_cargo.sii --- it is file where you add the trailer and 3d cargo in game list
goldhofer_mpa_k_boiler_parts.sii ---- that is the mixer file :) of trailer and 3d cargo file
def\vehicle\trailer\goldhofer_mpa_k\chassis.sii ---- that is the trailer
def\vehicle\trailer\goldhofer_mpa_k\boiler_parts.sii ---- that is the 3d cargo add for that trailer

all .sii filer are just description files link to 3d models into \vehicle folder
where you need to put all 3d files ... it is good practice to keep SCS file structure and keep all files on they places

i know all that look very confuses when you read it for 1 time but just read it look the files try to get the idea and see the files inside :)