Since the house has two 120v legs you'd only be powering half of the outlets that happened to be on the same phase as the outlet you plugged the deadman plug into. It's called a "deadman plug" for a reason too, be carefull...

An ideal access point would be the 240v plug for an electric drier since you could get to both phases with one plug. This assumes you have a 240v generator of course. If it's 120 then you'd need to figure out which phase the furnace is running on and plug into an appropriate outlet.