I tested my 2 6v golf cart batteries for fridge run time. At 6 hours, I had depleted them to half charge. So the most you could expect would be 8 - 10 hours of usefull power. Then you would be trying to re-cool the fridge and recharge the batteries at the same time using the "excess current" from your tow vehicle alternator.
I would open the TM and turn the fridge and fan off. Don't open the fridge and it will hold the cold overnight. The only casuality might be the ice cream getting soft. You can help it to retain the cold by freezing several bottles of water and keeping them in the freezer while you are overnighting. October nights should be rather cool and that should help.
I would not run the fridge on gas with the TM closed. If the fan stopped for any reason, you could end up with heat damage or even a fire.
MJL
