I don't feel qualified to answer specific Do-I?/Don't-I? questions on tax, but it's my understanding that if you have US domicile, you have to file US tax returns.
Domicile is the place an individual intends to be his/her permanent home; a place that he/she intends to return whenever absent. A domicle, once established, continues until the individual moves to a new location with the true intention of making his/her permanent home there. An individual can only have one domicile at a time.
I found a list online that gives a pretty good accounting of what you do to establish your domicile. By the reverse token, I think that if you DON'T do any of the following in the US, then you aren't domiciled there. (The quoted information below was taken off a military site and refers more to state domicile. But I thought the list was good and probably translated to our use.)
WHAT PHYSICAL ACTIONS MAY INDICATE MY DOMICILE?
One or more of these may prove your intent that a particular state is your domicile (numbers 1 through 11 are important – others are not required, but may help in a questionable situation):
1. Expressed intent, oral or written and physical presence, past and present (including duration).
2. Voter registration.
3. Vehicle registration as a resident.
4. Motor vehicle operator's permit.
5. Location of bank and investment accounts.
6. Explanations for temporary changes in residence.
7. Submission of DD Form 2058 (Change of domicile form).
8. Payment of taxes (income and personal property).
9. Payment of nonresident tuition to institutions of higher education
10. Declarations of residence on legal documents such as wills, deeds, mortgages, leases, contracts, insurance policies, and hospital records.
11. Declarations of domicile in affidavits or litigation.
12. Residence of immediate family.
13. Membership in church, civil, professional, service or fraternal organizations.
14. Ownership of burial plots.
15. Place of burial of immediate family members.
16. Location of donees of charitable contributions.
17. Location of schools attended by children.
18. Ownership of real property.
19. Home of record at the time of entering military service.
20. Place of marriage.
21. Spouse's domicile.
22. Place of birth.
23. Business interests.
24. Sources of income.
25. Outside employment.
26. Address provided on federal income tax return.
Generally, unless you have taken at least some of these steps, it is doubtful that your State of legal residence/domicile has changed. Failure to resolve any doubts as to your State of legal residence/domicile may adversely impact on certain legal privileges which depend on legal residence/ domicile including among others, eligibility for resident tuition rates at State universities, eligibility to vote or be a candidate for public office, and eligibility for various welfare benefits. If you have any doubt with regard to your State of legal residence/domicile, you should contact your Legal Assistance attorney for legal advice before deciding to change your domicile.
WHAT ACTIONS SHOULD I TAKE IF I CHANGE MY DOMICILE?
If you change your domicile, you must be consistent. If you are a legal resident of State A, then you shouldn't keep your driver's license from State B or vote in State C. Inconsistency is probably the single biggest mistake that people make in this area.