Re: mileage going down?
I don't know if you had this vehicle last winter or not, to compare what effect cold weather has on your mileage. Winter in a cold climate definitely reduces gas mileage, and it will be most dramatic if you go relatively few miles per trip. Then the warm-up interval is a significant portion of your total trip. The mileage in the warm-up interval is poor. The heater itself really doesn't have any real effect on mileage (yeah, it probably slows warm-up a bit, but not significantly).
In the South, gas mileage is the best in the long spring-like and fall-like seasons, dipping a bit in summer due to the A/C compressor load, but dropping the most in the heart of what winter there is (but warms up a lot faster than up North!).
On the other hand, if you've had this vehicle for years, and this drop is unusual, you're the best judge of that. Then maybe something needs attention.