These are mostly jokes about being a hick/country-bumpkin (乡巴佬）。 It is a common joke in America that tornadoes always seem to hit mobile homes (inexpensive, lightweight homes without a concrete foundation) and blow them away. So, that's the first joke: that the people in that small town need something to talk about while waiting for the next tornado to hit their mobile homes. The second joke -- about Opie and Aunt Bee -- refers to a famous TV show from the 1960s called The Andy Griffith Show, which is set in a small town with lots of hicks/country-bumpkins. So, the second joke is Dorothy's way of saying to the Clerk: "Wow, you are really a bunch of hicks around here." The third joke -- about keeping the big pot on the back burner -- is probably a joke that Dorothy is fat and would not be pretty to see in the nude. Putting something on the back burner (of a stove) means putting a cooking pot on the back of the stove because you don't need it right away. When Dorothy says she kept her desire to go to a nudist camp on the back burner, that's an idiom meaning she always thought about it but never acted on the thought. The clerk is saying: You should continue to not act on that nudist thought, because you are too big (fat) of a pot. The last joke -- don't you have a cousin you should be dating -- is a common joke about hicks/country-bumpkins that they date/marry close relatives (sisters or cousins, especially).

Oh! Gotta love this sitcom! And Betty White as Rose Nylund is my favorite. Watching American comedies is a big challenge for the learner though. It's real hard if you don't live there so can't pick up on cultural differences and nuances. So my advice would be, don't go too far with sitcoms unless your English is damn good. :) Back to your questions though. I think the clerk is jokingly saying that some can get busy talking while the next tornado is ruining their mobile home. This could be something like a trailer, a caravan. You should know that America is infamous for its tornadoes, right? And some people prefer living in mobile homes. If something is on the back burner, it is temporarily not being dealt with, especially because it is not urgent or important. The clerk is playing on words meaning that some big pots (some desires) should always be kept on the back burner, i.e. suspended permanently. Basically he's telling Dorothy to never try a nudist camp (because her look is not appealing). Dorothy is probably annoyed with the clerk because it's never good to date your close relatives like sisters, brothers, etc. "Don't you have a cousin you should be dating?" is a humorous, yet offensive way of her saying that. Hope this helps!