A really good sleeping bag...
I would agree. My main sleeping bag is rated to -10 (I wouldn't take it that low, because I probably wouldn't survive it even clothed). But the first seven day trip with it water bottles inside the the tent froze solid. Overnight. When the daily high had been in the 60's. Yet I was completely comfortable.
To keep warmth going you'll want to build a good bed of coals, and I think wood will work better for that than charcoal. But good sleeping bags, and maybe someone to curl up against, will be even more important. Once you break the $100 price tag you'll start reaching decent sleeping bags. Also, bring good thermal underwear (long sleeved top and bottom)...long johns don't count. You want synthetics and not cotton.