My 03 GMC Has idled for days at a time in the winter, and had no effects except on my wallet, Also this was when Gas was still under or near a buck a Gallion.
I'm not too sure about gas engines, but in a diesel it was always good to idle the engine at a little bit above normal rpm's to keep the oil circulating to the top of the engine, and to avoid cylinder wash.Volve engines are engineered so a person does not have to raise rpm's like this.
We have a combo smoke/C02 detector in our truck. We had never thought about it till our friends truck caught fire. They were NOT in it but it got our attention.
I idled my old chevy van with a 5.7 gas all the time and had no problems. She had 650,000 on it when I traded it in on my current van. I know the longest I ran it without shutting it off was the summer of 2005 I did not shut it off for atleast 4 1/2 to 5 weeks. I was running TX. to VA. roundtrips for Capitol One and I just locked the doors and left her idling even when I went into eat and take a shower. I can't stand hot weather. Oops I lied I shut her down once for a oil change but that was about it.
The Espar heater will run on gas or diesel and uses a tiny fraction of what you'd use idling. There are also 12 volt, roof mounted A/C units available.