The way I RP I ask first, Am I in an audible medium, a text medium, or a visual medium.
If in a text medium, I use no accent, and just use dialectal quirks that would be sensible with her upbringing, something like, referring to the Big4 by less then pleasant nicknames, since she might have learned to call them that before learning their official titles, or using the word ain't in normal speech.
If an audible medium, then I go really lightly on the accent, things like droppin' the g off active verbs, combined with the above it works well.
If a visual medium, then I can use a wee bit heavier of accenting since I can then use visual clues to fill in the linguistic gap if I worry there might be one.
A couple universal things: Quietly read over what you've wrote to make sure it sounds good and flows well before posting it. Never sacrifice looks for comprehensibility.* Accenting shouldn't be used all the time, there are times where even people with the most severe western twang or southern drawl to their voice will use the g on words. So consistency is not your aim, don't just go onto autopilot. Your goal is to make it sound good and look good. Everything else comes secondary and is effectively just flavour.
Oh, and grammar and punctuation people, grammar and punctuation.
*A caveat to the above. If the character is drunk, and slurring their words, its okay to make them slightly difficult to understand, since the characters would also have trouble understanding them. But only do this sparingly, it can be taken too far.