Make sure your air filter is clean.
Keep idling warmup short, get moving asap. I drive at/below 2000rpm till temp light goes off.
If you don't have one, absolutely get a block heater and use it.
Drive at the slowest comfortable/safe speed to keep wind chill from working cabin heater too hard.
Keep tires inflated to at least 35psi. (check pressure on a cold day to make sure)
As far as finding someone to diagnose fuel consumption issues? I don't know. Some of the guys on here with diagnostic tools may be able to help you do it yourself if none of the suggestions you're getting help.
P.S. it's -25°C this morn. Highway mileage wouldn't drop below 6L/100km until I lowered cruise speed by 10km/h.