I am a lower class white American, and most of my family lives in rural areas and many have owned trailers, and I have NO PROBLEM with white trash jokes. It is making fun of a culture, not a race. Even most "racist" humor is actually making fun of the culture and not the race (I am talking about the stuff that is actually funny, NOT "How many (insert minority here) does it take to screw in a light bulb, 6, because they are so stupid"). Matter of fact, a large portion of many black comedians best material is based on poking fun at black culture.
Of course George Carlin tops all those jokes with his observation about it, which I won't repeat her.