My second wife's sister Nell was one for that tornado anxiety too, of course she and the fellow she was married to till he went off to prison did live in a trailerhouse. Well they went to planning that shelter thing using a concerete septic tank, and when they had one delivered, think it was 1000 gallons, the park manager lit up like a neon sign.
Long & short of it was they wasn't allowed to bury that dang tank in the trailer park. Being a concerned fellow it hit me that sheltering from the tornado ain't near as important as having a place to collect your posessions and live after the storm passes, so I found plans on the internet to build an igloo out of cardboard and sticks. We all went over to the appliance store and rounded up a batch of fridgerator and laundry machine cartons, and rounded up a bunch of sticks too. Nell got the sticks all cut to size and mitered proper at the place she worked, and we preassembled some shelters. Then we took em apart down to panels and stuffed em into crawlspaces in several locations around town for just in case of a tornado.
If you notice when a tornado goes thru it hardly ever pulls up the floor so the crawlspace is a safe place to store your shelter. Now, here's the dangest thing, ever since we went and prepared ain't been a one of us hit by a tornado. Nell did have to use hers after her husband got locked up and she got evicted from the trailer house, but that's an entirely different situation. She did say it was real comfortable in there tillthe Park Ranger got on her case.