Thanks for those videos Pursuivant. I found the second one interesting - language and linguistics is a casual armchair interest of mine.
I cannot do any of those accents - a member of our squad is from western Oklahoma, and to my ears he sounds much like those people, if not using some of their regional terms.
I'm surprised you think that somebody with 'RP' could easily do a decent 'American Hillbilly' accent. To me that's almost like saying a Kentuckian can easily do a decent South Australian accent. (Not so long ago, RP was the required accent for announcers on our national broadcaster. Fast forward to 0:35 in the following video.)