aight fuck it
The word "white" in it, makes it a racial slur. It is not about poverty - it is, first and foremost, a term used against a single race of people. How could it be anything but a racial slur? I'm personally not too fond of the term. You can point out someone is poor or uneducated without having to mention race, right? There's trashy people of all backgrounds and ethnicities. Whether they be of the trailer park kind or the ghetto variety.
I think the point is that white trash are people that are white but still act like utter trash (thus implying nonwhites are trash by default). I personally consider it on par with calling someone a nigger. Even the term "redneck", while widely used among rednecks, can really piss people off in certain contexts.