Well Bird, thanks for posting this thread.
It prompted me to get my act together and sort out a couple of things to monitor how much gas I've got in my home BBQ and camper bottles.
It was never too much of an issue in the past, as I had 3 9kg bottles, one for the BBQ, one for a pizza oven, and one for the camper. Whenever the BBQ one ran out, I'd fill or exchange it, and swap that full one for the one in the camper. That way the camper one was generally almost full whenever we used it.
The new camper has a couple of 4kg bottles, so that method has gone out the window.
I finished up buying a Digitech luggage scale from Jaycar (on special for $20), and a couple of magnetic 'Accu-Level' gauges from Anaconda (about $7 each)
One Accu-level will live on the BBQ bottle and the other on one of the camper ones. They work by turning orange when you pour hot water over them, and then the section that the liquid propane is up to turns straight back to yellow when you stop pouring.
Oh, and btw. I wondered if a stud-finder would work on a gas bottle. Mine didn't.