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, ...
Good idea to have a CO detector. However, diesel engines produce very little if any CO. From what I have read there are some weather conditions that can have them make some CO. Maybe the amount of CO produced by the engine won't be enough to set off the detector.