A possibliity
My 1977 Cat 39 with a YSB12 had been on the hard for 2 yrs prior to my purchasing it. It had diesel smell, especially in the cushions. I cleaned the cushions but the smell still existed in the cabin. I eventualy found that the PO had used a standard auto heater hose for the fuel tank vent line. The fuel sits in the overflow/vent line for at least 6" from the tank. When the tank is full, The stagnent fuel was deteriorating the vent hose and the outer covering was absorbong the fuel and thus the smell. It was like having a glade air freshner filled with diesel oil. Very effective order producer. So, I replaced the hose with a good quality fuel approved hose. The smell disappeared. Check yours out. Good luck.John