I have a smaller propane tank, 11 lbs. When I'm running my fridge (small one) it lasts about 3 1/2 days. I cook out on a portable gas stove if I need to cook, to not use the propane up in the cylinder. The fridge does not require any power to run it. I often camp without power hookups and the fridge is designed to run without power, that's why it's 3 - way. So, my cylinder is just a bit more than half of a 20lb. I would think yours would last about 7 - 8 days if you're only running the fridge but it sounds like you use more for cooking.
I thought about taking another cylinder along but I don't want the extra weight. I just found and purchased an adapter so that I can hook up a disposable 1 lb. tank to the regular connection so that I can have a backup source of propane while I go and get the cylinder refilled. It's available at Camping World and a few other sources. They called it a steak saver, as it can be used to keep cooking on your grill if your tank runs out. I often wondered why they didn't make these, and now they do. I should have taken out a patent...
