[wingnut60]

The water method usually gives you an indication of where the liquid propane is--not too bad an indicator if your tanks are on the outside. If the tanks are in a compartment, not so good to have water on the floor of the compartment.
While it may not suit your purposes, I always run off one tank until it runs out--even if in the middle of the night. Then switch over to the full one, knowing if I don't get the first one refilled, the next empty tank is going to result in a very upset wife.