The term flyover state seems like it is unique to DCUM. DCUM was the first I saw it being used it, and is the only place I see or hear people use it. I think if you used it IRL, many people wouldn't understand the reference.
Trailer trash is extremely derogatory. Again, I have rarely if ever heard of someone refer to another person as TT IRL. Don't even see it that often on DCUM. Does an anonymous forum bring out the worst in us? I don't know. But folks who say this should be called out on it, because it is not okay. I'm liberal, but I would think a lot less of someone who called another person TT.
Redneck is interesting, because it is not universally thought to be derogatory. I grew up in the very poor rural Deep South, and some people there refer to themselves as redneck, and are proud of that label. My ILs grew up, and still live in this environment. They are solidly middle class. Not wealthy, but not poor (2 teachers). But I have heard my FIL call himself a redneck. Often when putting us down for living in such a high COL area, with the traffic, competition, high housing prices, etc. and, FWIW, my ILs are strong, lifelong democrats.
A lot of the white people who also attended my poor, rural Southern high school, and stayed in the down we grew up in also post "Proud to be a redneck" type memes on Facebook.
So, there is a certain cohort of people born and raised in the South that embrace the redneck label as indicating they have simple tastes, live in a rural area, drive a pickup truck and are proud of Southern culture and heritage.
Lots of folks think like this:
https://nicholsoutdoors.com/2013/12/28/rednecks-vs-city-folk-why-i-am-proud-to-be-a-redneck/
Redneck isn't a term I would feel comfortable using, but you don't know a lot about the poor rural South if you think it is universally an insult.