We recently returned from two months (9 weeks) in Arizona. We camped where we had electric 24 days and 39 days without. So, propane use for cooking only when on electric. We ran the furnace and water heater very little. During the trip we filled a 20lb tank three times. I haven't filled after the trip, but we didn't start with full tanks either. So something over 13 days of 'fridge and cooking per tank.
Last September we traveled to the Big Horn Mt. in Wyoming. We camped with electric 10 out of 25 days with three 20lb propane bottle fills. Same use as in Arizona but for the furnace, it was cold with lows in the mid 30F. Here about 5 days per 20lb bottle.
So for us, with little propane water heater use, we get between 5 and 15 days per propane fill.
I think the furnace and water heater are the big propane draws. Cooking and 'fridge aren't that heavy.
Our limits, with solar for electric, are the black holding tank and the fresh water tank.
Your mileage may vary. :-)
2011 Escape 19
1955 Shasta 1500