How do I repair my damaged truck?

Just like in real life, you can't fix your vehicle with the press of a magic button. You have to get the vehicle to a service center. Each city usually has one, as indicated by a red square with a wrench symbol.

If you don't wish to drive to one, you can, alternatively, press F7 and then Enter to be towed straight to the nearest service center. You will be charged for the towing though as opposed to driving yourself to one. So it comes down to fuel costs and real-world time vs towing costs.

When parked at a service center, you will see the following:

And you will then be given the option of what to repair, with a repair button at the bottom:

You cannot repair a damaged trailer though.

... unless you edit a saved game.sii file - then you can 'repair' each item (engine, chassis, trailer, etc.) Any damage will be listed as 'wear' ... edit lines that start with wear: & (followed by an 8 character hex number, beginning with 3) to read ...

wear: 0

If you've 100% damaged anything it will read

wear: 1