I have always understood -- and used -- "redneck" to refer to shamelessly bigoted white guys, regardless of their economic status or where they make their homes. Which is to say I would not hesitate to so characterize Donald Trump, who is neither poor nor rural.
I'm afraid the history of the word is clearly tied to classism against impoverished and lower-income rural white people. There has been an attempt in the past few decades to associate the word and low-income rural white people in general with racial bigotry, but I would argue that that is itself an example of classism -- racial bigotry is a big problem among white people of every level of socioeconomic status, not just low-income white people.
If you mean to talk about bigoted white people, I would urge you to just use that phrase.
Not all rednecks are bigoted.
And in the end, most rednecks don't mind being called by that name anyway (as any Jeff Foxworthy fan knows), so how offensive could it really be?
I would characterize the use of the word "redneck" among some poor and low-income rural white families as an attempt to reclaim a word that has been used to marginalize them -- the same principle being at play among some black people who try to reclaim the N-word. I would argue that just as people who are not black should not in general use the N-word, folks who are not impoverished or low-income rural white people should not in general use "redneck."