That is the question looked at by researcher Graham Phillips from the UK. He has written many books on the subjects of King Arthur and the Knights Templar. He also has done much research and has concluded some very ancient ruins built in New England were done by the Celts at the time of Arthur (the 5th to 7th Century).
I always thought Avalon was mythical or an idea of what a place should be. I never thought about it being a real place, and if it was in the Americas, then it would of been an Eden like place for those who went there. The natives were not hostile at the time, and there was plenty of bounty to be had.
What do others think of this idea?