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, ...
While you should not have any smell in the coach, what your engine/pollution control system is doing is regenerating the diesel particulate filter. Your last post about the timing and length of time of the odors are consistent with DPF regeneration.
You need to have this coach inspected both by Freightliner, to make sure the exhaust system is intact, and by Fleetwood who needs to recheck and reseal all of the areas around the engine.