On the BC ferrys, a deckhand will give you a yellow sticker to wrap around the valve that says you've turned off the propane. They of course can check for that sticker (and a closed valve) anytime.
Bill, Kathi and Zorro; '05 Beaver Patriot Thunder
2012 Sunnybrook Harmony 21FBS (SQEZINN)
2007 Jeep Grand Cherokee Overland