To get an idea , based on your usage, my suggestion would be to fill both tanks, then operate on one only until you run it out. Then you will know based how long your showers are and how you cook etc you can expect to go on two full tanks.
As far as running in the winter, If electric is available, thats the way to go. If you have kids or a dog you may want to look at oil filled portable heaters, a little safer I feel. If your dry camping look at the small propane units, a Lil Buddy puts out a lot of heat but will burn about a pound of propane every 6-7 hours on low, and every 3-4 on high.