I think the point nearly everybody seems to have missed is that there's a huge difference between the furnace being switched on for 8 hours and actually running for 8 hours.
Set point is also a huge factor, at 60 or 65 mine will only cycle 4 to 6 times per night, even with only 2 marine deep cycles 2 nights in below freezing temps is a cake walk. That's only a total of maybe 2 hours of furnace run time.
If the OP was to also use some engine heat as my neighbour to the west suggested I can't believe there'd be an problem at all.