Your future in-laws are crazy if they think a trailer is safer than a house in a tornado. Absolutely. Crazy.

Are you prepared to live on your own? If he wants to buy a trailer, let him. Let him go live in it, and you can stay in the apartment. I'm not sure how far along in the wedding planning you are, but you might want to investigate a prenup so in the case that you do marry him and eventually get sick and tired of his family, and divorce him, you won't have to take on his debt. At the very least, letting him move out would probably send a pretty clear message to everyone involved that you will not be bullied into something you don't want.

Dude, seriously, a trailer in a tornado?