i dont want to wake up and find out its 45degrees inside! i wanna know at that moment its empty.
How about the simple solution. An alarm that goes off when the temperature reaches say 55 degrees, or whatever you set as the alarm temp? Also, as a RV owner myself, I have both tanks connected but only one turned on. When one goes empty, there's no need to switch tanks in the middle of the cold night. Just turn the other one on. The next day or so I swap out the empty tank. The only time I have both tanks on is while traveling, so the refrigerator will continue to run even if one tank runs out.
Another possible way other than a simple thermostat alarm is something that's connected to the furnace. When the furnace tries to fire up and it can't because there's no fuel it trips some sensor which you monitor and that sets off an alarm. But thinking about the last time my tank ran out while I was sleeping, I seem to remember the starter on my furnace clicking a bunch, and that actually woke me up. If you have a pilot light, that may be even easier, just monitor the temp of the thermocouple. When it's no longer hot, your pilot is out which means you're either out of fuel or you have some other type of furnace problem, which would warrant an alarm too.
Good luck and keep us updated!