I'm glad you started this thread as I have been thinking along these lines due to recent and ongoing Tornadic events.

Some of my thoughts:
Tornado shelter approximately 15-20ft from the house. Consider this a mini-max.

A shelter large enough to store a case of bottled water, 2 cases of MRE's, all of my camping gear, my family and my immediate neighbors.

A camper able to accomodate my family, stored 10-20 miles from my home.

Because of my personal circumstances, i am looking at an above ground tornado shelter. This would accomodate the above requirements, would be easy to wheel my DD1's wheelchair into, and are fairly cheap. They are made and sold by concrete companies and septic tank companies. They also have a steel lockable door and passive ventilation system.