As a white southerner, I always understood that the term "redneck" originated with farmers and other outdoor workers who had sunburned necks. It tends to connote someone more rural than urban, not wealthy, unsophisticated and conservative. I've used the term with a smile to refer to my own heritage. It's one of those things you can call yourself or a close friend in fun but that you wouldn't call a stranger unless you're fond of fighting.

"Trailer trash" seems offensive to me and I can't imagine using it.