I have noticed that after a period of driving our new Fleetwood Expedition 40x there is diesel exhaust smell that can be noticed in the front cab. It doesn't happen when first getting in and driving, ...
The DPF, not DEF, is what makes the exhaust not smell like typical diesel exhaust. It still has a smell do it, though it's not very strong unless the DPF is going through regen.
If you are smelling typical diesel exhaust smell then there's not really anything else that could cause this other than an exhaust leak prior to the DPF.
Hopefully Fleetwood will be able to locate the problem quickly!