Tent is irrelevant for warmth, unless you're bringing a canvas tent and hot tenting.
As someone mentioned, the temperature rating is for keeping you alive, not comfortable. The lower the rating, the more expensive the bag. You can double bag. Put your nicer bag in a cheaper bag and you'll be golden. Also, hot water in a water tight Nalgene by your feet does wonders. You could also throw some hot hands in socks and go to sleep wearing them. Beanie that covers your ears will keep you warm too.
Don't sleep directly on the ground, on a cot without a pad, or on an air mattress that is directly on the ground. You need insulation from the cold underneath you. I usually do a cot, a backpacking pad that has foam and air, then sleeping bag(s). A buddy heater is great for knocking the chill out before you get up in the morning (or night to pee).