Really bad MPG since winter hit??

[racingbmwm3]

would need to know the efficiency of the alternator, but let's say you have front and rear heated seats, each pulling 15amps at the high setting? ~200W/seat (@14V). ~800W total. If the alternator was 100% efficient, that is about 1.1HP. even assuming a horrible efficiency of the alternator of maybe 50%, that's 2HP. Same logic goes for the heater, but the heater has even less impact since the fan is the only (large) electrical load. The heater does have an impact on the length of time it takes to heat your engine up to operating temp though.
Which is the main reason for lower mileage in the winter. Longer warmup cycle, meaning the engine is running rich for a longer period of time. Combined with longer warmup times for the oil in the engine, transmission, transfer case, and front and rear differentials. That's alot of sources for increased drivetrain losses due to thicker fluids. Additionally, the cold air isn't ideal for a complete burn, but once warmed up, your O2 sensors account for this and correct it as much as possible.