VeeDub, I couldn't get the link to work. I tried to navigate through the page to find the pdf you referenced, but it's in Russian and a little cumbersome after translation.
If the heater is 300W, on a 12V system that draws 25 amps. To me, that is not negligible. If the heater only operates below 0degF, what would explain the drop in fuel efficiency during cold, but not sub-zero, temperatures? The same winter blend fuel that I get 40+ MPG on warm days will only deliver ~32 in the cold.
Urea freezes at 12 degrees F, so the heater system will definitely be operating at anything under 12. I can't find the documentation right now, but there was technical documentation from VW describing the function of the urea-scr system in the Toureg, and I imagine the system would be similar. At anything under 20 degrees if I am recalling correctly, there is some heater functioning within the urea tank. If it is at near 12 or below, there is one heater that immediately thaws a small reservoir of fluid, and then a general heater that continues to heat and thaw the rest of the tank. It's done this way so that there is at least a little bit of liquid urea soon after start up. Additionally, the urea lines are all heat traced.
Additionally, as it gets colder, there is more general friction on moving parts. The transmission, wheel bearings, engine, etc. will waste more energy due to increased friction until the heat from that friction is able to warm the lubricating greases and oils. And yes, 20 degrees vs 40 degress has a big impact on this. Plus, you are probably running the heat longer.
All this could easily lead to an mpg drop from ~40 to 32 mpg. Especially if you have more stop and go traffic on your commute, as it takes a while for the diesel engine to warm up. A few weeks ago my mpg on the way to work was around 45 or 46 according to the computer. This is when we had a warming spell of 50 degrees. This morning it was 4 degrees. My computer said I was at 40 mpg this morning. I'm not suprised at that at all. And my commute is about 2-3 mi surface streets (but constantly moving) and then about 13 mi highway.