It's impossible to say how long your battery should last. How long it will last depends on its overall capacity and condition and, of course, how much demand you place on it. There is a small current draw all the time for parasitic loads (such as CO detector and clock/sound system). The refrigerator will draw a small current for the control board and the gas ignition. The refrigerator also draws current continuously to power a heater in the door to prevent condensation in humid weather. Some refrigerators have a switch to turn off this heater. Turn it off to significantly extend your battery. The furnace fan draws a lot of current. If your interior lights are incandescent, they will draw a lot of current. Switching to LED's will save a lot of power.
Something else to consider: The length of time it takes to recharge a depleted battery not only depends on how depleted it is, but also how much charging current your charger produces. Many campers have WFCO converters as OEM equipment. Many of these don't seem to work as they should. They have a tendency to work only in the absorption mode (rather than automatically switching to boost mode). If your converter is not working correctly you can replace it or use an inexpensive stand-alone battery charger that outputs 10 Amps. If your converter is stuck in absorption mode it can literally take days to recharge a depleted battery where as a 10 amp charger will recharge it in a matter of hours.
