Is there more then one tank ? If yes, then just keep one shut off. When the first one runs out you will know how long your propane will last.
You might consider not heating the hot water tank all the time if you believe you will run out. Maybe shut it off at night, or run it every other day. You can always heat up a pot full on the stove if needed.
My first pop-up had one of the 5 gallon hot water heaters and I found I could just leave the pilot light lit and it would heat up the tank overnight - but that was before the auto-ignition types we have today.