We've never had a problem like that and typically dry camp at least one night on our way places. Are you certain everything else was off? We've dry camped with the furnace going for two nights on fully charged batteries before they went close to dead. Also, are you certain the batteries were fully charged to begin with? Don't go by the lights - they're next to useless. Get a hydrometer and test them. A decent one (not one of those little pocket thingies) costs less than $10. One of my next purchases is going to be a real battery monitor (Trimetric), even though we may not go solar.
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