Defeats the purpose the OP stated
In another life, I worked as a death investigator and came across many individuals whose deaths were as a result of hypoxia caused by exposure to carbon monoxide (CO). The problem with CO and the much misunderstood realities of being exposed to it, is simply that it is not just a fresh air issue. If you are exposed to CO for an extended period and breathing the air(fresh or otherwise) that it is in, it will ultimately kill you. Unless the "fresh" air is being blasted past your nose and mouth and thus blowing the CO away, having passive non-moving fresh air around you doesn't help you one little bit if there is CO in it.
Very simply, it is a human physiology problem with regards to the takeup of Oxygen (O2) by the hemoglobin in the blood from the air in the lungs. Again very simply, the O2 molecule is more "complicated" than the CO molecule. So, if the hemoglobin in the blood is offered both O2 and CO in the same air, it will automatically take up more of the CO. After a while with CO saturation in the blood, the hemoglobin is unable to take up any oxygen and the exposed individual will succumb due to a lack of oxygen (hypoxia). "Carbon Monoxide Poisoning" is a misnomer. CO is known as the "silent killer"(for good reason) and is both odorless and insidious.
There is nothing worst than a nagging Nellie, but my advice is that if there is anyway of avoiding sleeping in a vehicle that has an under body exhaust system and has the motor running, take any other option.