When I had my 23' Winnebago, running the furnace all night would deplete the battery in 30-degree temps. It's the fan that sucks the juice. I replaced the single 12v battery with 2x 6v batteries. If they got a full charge during the day they handled the furnace without problems.
Now that I'm in a bigger rig, I'm worried my furnace won't keep up with the bigger space at night.
(Our annual October trip (week after next!) is boondocking for a week high in the SoCal mountains. 30's at night, 80's and tons of sun during the day. Solar and 6v batteries work perfect in this situation.)
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