You claimed the truck was filthy, and stated you refuse to drive it.
You return to the office, and the boss comes out, which I assume he thought you were over reacting, then he fully agrees with you.
I can’t see what you did wrong, your boss backed you and was rightly annoyed that an expensive vehicle was trashed, by a driver who is obviously an idiot,
You did the right thing, don’t worry about the other driver, he has no respect for you or company property.
If I were given a new truck it would be kept ■■■■ and span . . Why, because it would be my home… even if I was a day driver with a truck I would keep it ■■■■ and span, for personal price and self respect.
You did the right thing.
Playing devil’s advocate a bit but all replies in this thread are wrong.
Yes you should feel guilty as you have made a code breach, the unwritten rules that where possible all drivers in the same yard will look out for each other.
Even worse if you didn’t have to sleep in it.
Yes it’s disgusting to tolerate another man’s filth and we all have levels of scum we can put up with but if it was me I would have done my best to clean it and if the gaffer should wonder why I’m doing it that’s up to him although I wouldn’t have made it obvious.
Consider yourself marked as a gaffers pet.
I’m reminded of one time I had to night out in another blokes truck, it was minging but I cleaned it best I could and coped, said driver was a mate who was having a tough time in his private life.
Each to their own but Jesus lads… 
(For context bear in mind my own truck although clean to the naked eye looks like a ■■■■■■■ crime scene under a black light).