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).