Most D&D campaigns don't even pretend to be medieval. Most are anachronistic messes of early renaissance to age of sail with random items tossed in from the dawn of civilization to steampunk victorians. Not to mention settings that aren't historical at all,like Dark Sun, Eberron, Planescape and Spelljammer. I guess if you are looking at high fantasy campaigns you get more medieval elements but that's less about history than it is about Arthurian influences and Tolkien.