You can find stuff guys have posted online, or even from the dealer sites.
At Peterbilts site, you'd go to the Peterbilt
Life tab, and click on
resources, and you will find some manuals.
..:: Peterbilt Motors Company ::..
scroll down and choose a low/or fast download for some pdf files.
If you need more specific technical drawings, like PACCAR body manuals, you might have to order that through a parts dept.
I haven't searched Freightliner stuff. Do you have something specific you want?