Propane bottles keep a constant pressure inside till they are almost empty, so a pressure gauge is a joke. Google propane level check for a variety of ways to check level, but realize that the only correct reading is by measuring weight. I have two bottles and keep one closed for two reasons; one, the auto change over regulators can fail; and two, I want to know when a tank empties without constantly looking at a rather inaccessible flag.
A 30 lb. bottle lasts me about a month with some cooking and some electrical hookups. YMMV
