I was wondering, if a person is applying for disability and has a mental illness in addition to physical problems, does his or her doctor need to fill out both the physical and mental RFC forms?
Ideally, when a disability applicants has a mental impairment, a treating psychiatrist should fill out the mental RFC form for you. Notice I didn't say psychologist, because psychologists are not doctors. Administrative law judges (ALJs) tend to consider opinions that come from M.D.s, which psychiatrists are, more valid than psychologists' opinions (though this isn't true in the case of mental retardation, borderline intellectual functioning, or learning disabilities).
If you've only been seen by a psychologist or even your internist, family doctor, or primary care physician, have this individual fill out the mental RFC form. But if you have a chance to visit a psychiatrist before applying for Social Security disability, this can be a good idea.
For your physical impairments, try never to have a mental health professional complete the physical RFC, even if the person is a doctor (psychiatrist). You should be seeing a doctor who is familiar with your physical illness, injury, or disease, and perferably one who specializes in it.