When I bought my boat in Panama there was a propane tank connected up, and a spare. The gauge at the tank showed it to be near empty. I took it to a local guy who fills tanks and he picked it up and said it was nearly full and the gauge was wrong. I was still using it when I got to Fiji six months later, gauge stlll shows near empty. Who knows how much longer it will go before I need to go to the spare.Excellent point.
I was a little bit excited by that product, the first time I saw it. But I cook and bake a fair amount, and after eight months on board, I'm still surprised by how little propane I use. Although I'm cooking for one, not a crew of six, so maybe not all that surprising. I've changed out one of the ten pound cylinders in that time. Oh, and maybe three or four of the little bbq cans. (But some of those might have been due to not getting the valve closed fully.)
BTW, there are much cheaper, if not quite as polished, versions of the vacuum solar oven available. You can even buy the tubes and roll your own if so inclined.