Good question B&W. You are wise to be wary of the heat. We all lose our ability to handle the heat as we age, and heat stroke is accumulative. The risk is real, especially if you are alone, as you will not notice what is happening to your body. Heat stroke negatively affects our alertness and comprehension.
I would not idle an engine for hours like that. So what if the cops do it - they are not concerned one little bit about engine longevity like us plebs need to be! If I was you I would get a separate little AC unit. U read about parallel systems? It is what they do in aircraft. separate systems for separate uses, and save the main system for it's primary purpose. The risk of exhaust fumes is real, especially if you wanna have a rest. Get a rooftop separate unit or some little generator powered system outside the vehicle.
Have you got some ice-making capacity? Check out this-
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They are superb. hang them around your neck also if you want to, and get an ice vest also. Vital for when you have to get out in the heat and change a tyre or something. Its not worth being a tough guy and saying "She'll be right mate, only take a few minutes..."
This guy is Australia's best heatstroke expert. Have a read up-
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Just a good old wet towel with a fan blowin' on ya face can be good enough most of the time, then use the AC when you really have to. Keep your feet in ice water too, can help a lot. Most important is to cool your core body temperature down after a hot day behind the wheel or working in the heat.
have you looked into a second roof on your RV, like the old Land Rovers had. That makes a huge difference too, to internal temps.
see this photo
There's more than one way to skin a cat.