Concur with the thermostat, it controls the flow of the coolant to the radiator to maintain the engine temp. Are you sure you got the correct temperature thermostat when you replaced it? Regardless, they are cheap, they can go bad in a short time and stick open.
Cold windows will have humidity in the air condense on the cold surface, i.e. fogging windows. Note, when you turn on defrost, it engages the air conditioning to remove humidity from the air and stop the fogging of the windows.
That is why you have combination air flow settings on the environmental controls, if windows are fogging while you're using the heat, then switch to defrost/panel or defrost/feet, to stop the fogging.
Low mileage, the engine running cold, under a certain temp will run richer, hurting mileage. But depending on your driving habit and conditions, but 13mpg in a Commander, even a v6 is NOT out of the ordinary.
Winter, mileage usually drops for all vehicles, most areas switch to a winter formulation of gasoline that has more oxygenation in it, reducing mileage and your engine start much colder and take much longer to warm up each time you drive, which the A/F ratio is much richer for a colder engine and doesn't get back to normal before the engine gets above a certain temp, somewhat before full operating temp.