Cummins ISX15 cold idle?

Been searching the net and can't seem to get a clear answer. What RPM should I bump the cruise up to while sitting idle when sleeping in cold temperatures. Its about 5degrees out tonight, supposed to drop to -1. I have a ISX15 CM2350. Owners manual only says idleing for longer than 10-15 minutes is no good for the motor. When I turn the cruise on and hit resume it goes to 700rpm. Good enough? Seems like from what I've been reading people say 900 is good? I do have a APU but doesn't seem to keep up in these very cold temps.

 

I'm not to familiar with the mechanical aspect. From the sounds when you idle it at a higher RPM its burning the fuel off by keeping the engine hot versus at a lower rpm not completely burning it and clogging up?

 

Yes I own the truck. I have a new dynasis apu that I am currently having issues with the heat on it therefor need to idle until I get the issue with that fixed. I tried idleing last night around 900 and couldn't stand all the noise it just didn't sound right at all. Now the Engine is whistling. If I idle from now on it's going to be at standard sitting low like I've always done with this truck.

 

Stand alone.