We dont even get that cold here but I went ahead and did a cheap vevor diesel heater with 1.9gal tank in it last week as the high 
tops are harder to keep warm.  My test last week in the 40s showed it kept 20+ above ambient outside on setting 3 or 10 so I am feeling good this year.
Last year we got some 15-18 degree weather which almost never hits us in West Texas that cause me lots of sleep loss and even put 2 electric heaters in the high top on different circuits to fight that odd all cold weather here.
That was the graph of my test with it being about 42f overnight.  Dropoff at 7am was me with the doors open and heater off checking it out.  Really easy to install in 10-20 min and total cost was about $150.
 
