No, it's not normal. The 12V should last you all night with the CPAP and furnace as long as the furnace isn't running all of the time. I wouldn't expect it make it two nights with both items drawing power at night. As said above, two 6V batteries will do a lot better but you still need to figure out what's wrong with the power usage before buying two 6V batteries.
Yes, leave the refrigerator running on propane. The 12V control panel doesn't draw enough power for one night to worry about. If you're going to use the furnace, then you have to use its fan, it won't run without it.
If the battery is good, then there's a problem someplace else.
Bill