At night there is typically less wind (at least in my area) which could make it easier for exhaust to accumulate in area of RV rather than being blown away from RV.
Modern automobile engines are extremely clean and don't produce much CO. Most generators, lawn mowers, boats, etc. don't have automobile-level exhaust treatment and therefore concern me more. Modern cars can be so clean when operating properly that their exhaust is reportedly cleaner than the intake air in some cities. Unfortunately generators don't have that level of emissions control.
I knew a mechanic who worked with my dad who died while sleeping in the back seat of his car with the engine running to stay warm. That was a long time ago when cars polluted a lot more. That's not to say it would be safe today.
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