I could be wrong, know zip about Medicare plans, but I think most U.S. health care plans will cover you in the United States. (I think that is the case with BCBS -- my carrier). Usually coverage stops at an international border. Some plans will reimburse overseas health care costs, others not. Read your policy.
...BCBS coverage is statewide only, whereas Aetna is nationwide.
When i'm on the road "Blue Cross" covers me throughout North America.