Trailer parks are stereotypical places to get hit by tornadoes. Tornadoes don't hit trailer parks more often than anywhere else, but since trailer parks are often wide open spaces with few trees to break up the wind, and a lot of loosely-secured objects to become projectiles, they're often hit hard enough for the news to do a story on it, thus perpetuating the stereotype.

The joke is that since we've just determined that dinosaurs die from tornadoes, we should look for fossils in the stereotypical tornado place. For comedic effect it overlooks the fact that trailer parks are cleared of trees and filled with clutter by humans, so the locations wouldn't be any more likely to have dead dinosaurs - even those killed by tornado - than anywhere else.