You will not make it 8 hours a night with your furnance on a dual battery setup. Best advice I can think of giving is to try to run it before bedtime and then bring lots of warm covers and sleeping bags for the nights. You should be fine in the 40's. A hat is also good on your head when sleeping. Run your furnance in the morning to warm up the trailer before everyone gets up.
I would ask your neighbor if you could run a 20 amp line over for the night - maybe use an electric heater at night.
We have made it through a lot of cooler nights in our trailer - it is part of the experience!!