It also doesn't help that my expertise is in hurricanes, which only sometimes involves tornadoes, which are two very different beasts with a whole separate list of prep and problems to fight. Especially as "tornado alley" really isn't a place that has to deal with flooding, which is the only reason why most tornado shelters can even be built to begin with.

Anything that can beat a tornado will probably kill you in a flood, so you can't really set a standard for both.

Also one of the reasons why most of the South is trailer parks is because trailers by definition and default have a baseline flood resistance by simply being 6~ feet off the ground and designed to be moved around a little. Only the richest can afford actual houses, and that's because you have to not only afford the house but the entire acre+ of land to place it on with a literal hill you will have to build it on top of.

The South isn't hurting for space or land, so there isn't a need to build giant apartment complexes that go up, so instead they just spread out in a lot for all the people stuck renting still. Until they can finally afford to put a nice trailer on a plot of land all their own.