Most APU's burn 1 gallon an hour compared to 10 gallons for idling. Or some such number. You can turn the apu on manually or it can be set to turn itself on when the batteries drop to a certain level. Because they are all a little bit different, I would suggest you google what ever apu you are looking at and download the information there.
Glad I don't run your APU or idle your engine, I'd be broke. Try about a third of a gallon, for the APU and a gallon for the big motor.
I thought it was more like a gallon overnight? As far as powering on, ours has a panel that you 1) manual power on, OR you program in when: 2) it hits a temperature you specify 3) it hits a day/time you specify 4) it detemines batteries are low.
I was told my tripak drinks 1/10 a gallon an hour on average. Sent from somewhere out there on the road.