Your fans cycling properly? One thing to check.
My wife (like a lot of people, I bet) doesn't realize that some positions on your control make the A/C compressor and radiator fans run even in cold weather, like the defrost position and the split defrost/vent position.
So she drives around, complaining about lousy mileage and lackluster heat from the vents, when she's running the A/C and fighting it with the heater, and paying to run an engaged compressor much of the time. Those fans running at the radiator when the compressor engages cause the coolant to run unusually cool, further suppressing the heater's ability to work properly.
Every season I have to "re-train" her about this fact.
The reason they do that is the drying effect of A/C air will clear moisture from inside the cabin that's trying to condense on the cold windows, making the defrost more effective, and it allows the compressor to run at times in the off-season so you won't find it locked up in the following summer from non-use.