Are you getting exhaust fumes or raw diesel fuel fumes in the coach?
When we first got our present coachm we could smell raw diesel and I found that the fuel return hose for the generator had a small hole in it right where it slipped onto the fitting at the generator. Evidently they overtightened the hose clamp at the factory and it cut the hose. It only leaked when the generator was running OR if I lifted the hose.
If it is exhaust fumes, you should check the clamp where the exhaust tip clamps on the pipe at the after the muffler and also check for cracks in the exhaust pipe.