I think your 865 has the same furnace as my 825. It only draws 1.8 amps, so I've never had a problem running the heat all night at below freezing temps on a single group 27 deep cycle. Thermostat set to 55 or so. We've done it consecutive nights with just the truck charging in between.
In the interest of full disclosure, I also made the error of trying to run the fridge on 12V during a full day of running around shopping. Killed the battery dead by the end of the day. Never did hold a charge as well after that. Live and learn.