Our results are similar to Wayne's.
In really cold weather with some conservation of use - i.e. turning the diesel off during daylight hours when the sun warms the coach - I calculated that we might burn about 1 gallon a day.
The furnace would run about 2 mins at a time, and depending on how cold it might run every 15 mins or even every 7 mins when really cold. With a fuel run rate of 0.5 gal per hour, you can calculate how many minutes the furnace might run over a given 24 hour period (or 8 or 10 hour if that's all you're leaving diesel on during a 24 hour period).
Without being so careful (i.e leaving the diesel off during the day) I figured usage would be more like 1.5 to 2 gals a day.
We also used about 40 gallons during perhaps a 35 day period, and this included some VERY cold weather (teens at night, some days only in the 30s).
Advice - if you are going to be spending a month or two in very cold weather, top off your tank before parking! If more than two months, make arrangements for fuel to be delivered.
36ft 2006 Alpine Coach FDDS