I have friends in Indiana, one of whom became a teacher in the early ‘80s. I talked with him after he’d taught for a few years and a couple of different places, and he talked about how the kids in the farming areas were dramatically more polite and well behaved than the kids in a town of about 20,000 where he grew up and taught for while.
I think it is regional, and urban/rural is also a big factor in any state.
Short answer to the OP is, yes, Southerners in general are quite polite, depending on where you are. With that said, there are polite people everywhere, you just have to find them.
Some of the nicest folks I have ever encountered were in Portland Oregon. Very polite, courteous and had the same respect and manners for others I see living in the South.
Two of the worst places I have ever been for manners were Los Angeles and New York City. I think this has more to do with it being urban than where it is.
Now, with that said, I will tell you that in NY and LA I was told in no uncertain terms was I to LOOK at anyone walking down the street and especially in cars. In the South, even in larger metropolitan areas, it is quite common to nod or wave at folks in cars or on the street. Even in Atlanta, I saw people holding doors for ladies, saying hello to each other generally being nice to one another. Not saying other areas, like LA or NY would be devoid of such politeness, but I never saw it.