If you are coming from other forms of camping you will think that your TCer is a 5 star resort even if you cannot run your furnace all night.
Honestly in the cool weather I do not run my furnace very often, in fact I sleep with the vents and a window open and I am still warm and toasty. I do turn on the furnace in the morning to get some heat into my old bones but that is it. Doing it that way, and with my LED lights I can stay out for a week with just the one group 31 battery (was about the same for the group 27).
Like was mentioned make sure that your truck battery is isolated from the camper, can be a problem when you go and turn the key and nothing happens.