John (the OP),
Remember that your RV's propane furnace should not run continuously during the night - it cycles ON and OFF all night. This means that you don't just divide the amp hours of your battery by the amps that the fan motor uses to get how many hours the battery will last when running the furnace. For instance, if you furnace draws 7 amps when running, if it's on about 1/2 of the time during the night it is only drawing, on average, 3.5 amps per hour - not 7 amps per hour.
The better you set up your RV to not let heat out and to heat as little bit of it as possible, the more your furnace will stay OFF relative to how many hours it will be ON.
Ron Dittmer's post above explains the advantages of heating a small, RV with no slides ... and he's right on.
We have only a 24 foot motorhome and always run the furnace all night set at around 65 degrees - we don't like to sleep with blankets piled all over us. We block off the cab area in front, so the necessary volume to heat inside is cut down to a very small amount. We also carry roll-up plastic bubble window material to additionally insulate all windows on very cold nights. This is more effective than double pane windows. We really appreciate our RV's propane furnace for the complete comfort it provides - it has multiple outlets throughout the RV, so even the bathroom is kept nice and warm.
That being said, I recommend at least two Group 27 batteries in your RV for a reliable 2-3 nights from your propane furnace, before each recharge.
