I'm not aware of what type of machine that you're using Paul, but here are things we've found in our service department that contribute to high levels of carbon monoxide in the truck:
1. Exhaust of slide in unit put through the floor or step. It is really nice not to have that hot, carbon monoxide laden air blowing directly at you while you start or adjust your machine...but.....the fumes tend to role under your truck and back up into the van and passenger section. Let that stuff blow out, and if you don't want it in the building or dwelling, park your truck in the other directxion.
2. Loose exhaust pipes, or holes in the exhaust pipe inside of the unit. You might be surprised how many we find in this condition. Cleaners adapt to the noise, and often don't realize it. We mostly find this during oil changes or other repair requests.
You seem like one of the guys who wouldn't miss those two things yourself, but it is worth mentioning to others who might be using that type of exhaust redirection, or who haven't "peeked under the hood" for a while.