Windows notepad, or notepad++ (free download) to edit def files.

And the SCS extractor to extract def.scs to get the def files you need to edit.

Thanks, i already have those to edit truck def files, but im not sure what/where to look for the files for the ownable trailers. I've only edited mod trailers.

in the cargo files you can see what trailers are used to haul specific cargo, for example:
def\cargo\largetubes

and in Trailer_cargo (def\vehicle\trailer_cargo\scs_log) you can see what cargo's are made for the trailers.

Another thing you can try is add logging cargo to more companies' in/out directories.
eg.

def\vehicle\trailer_cargo\scs_log contains these cargoes:
beams.sii
logs_long.sii
logs_short_2x4.sii
tubes.sii
utility_pole.sii

whereas

def\company\dg_wd_hrv\out contains:
bulldozer.sii
digger1000.sii
woodchipper.sii
wshavings.sii

None of the log cargoes above appear in dg_wd_hrv' out directory.
You could copy the two 'logs_' files into that dg_wd_hrv directory. This will generate more jobs in that company everywhere on the map.
I'm sure there's other lumber/timber/wood products companies that you could copy them to as well, theoretically.

.. or
you could try Xyzan's Virtual Speditor mod.
any cargo - or specific cargo only - from any company to any other company anywhere on the map.