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To be sure I understand:
Technically, if you don't have a US address, you don't qualify for Medicare anymore.
If you do qualify, all "Medigap" policies, I think, cover emergency needs when travelling in Mexico.
Check AARP's website for the UHC policies it offers.  For a given type policy, A, B, C, etc., all companies offering the policy must have the same coverage.
If you don't qualify for Medicare anymore, there are 3 options, Seguro Popular which costs nothing and provides broad but not unlimited coverage, IMSS (Seguro Social) at about $300US/yr or private insurance which will cost up to about $300/mo. and doesn't, last I checked, cover pre-existing conditions ever.

There are also the doctors at Farmacias Similares for questions and concerns.  My experience is they are competent.

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Thank you.  I am going back to Texas the end of November and will go to the Social Security office and find out if I lose my benefits if I do not have a USA address.  I was under the assumption that I did.  I have part B and with that and my San Antonio, Texas address I have had a supplemental policy that has great coverage.  My costs are negligible.  My plan was to return to the USA if I had any major illness to take advantage of my medicare and was hoping to find a new company for the supplemental insurance if I were to return for medical treatment.  I will let you know what I find out if you are interested.