For completely personal reasons I have talked to numerous low echelon military people in the last decade. I can't recall one of them who sounded illiterate, although some may exist. There may be a higher percentage of illiterate teachers in this country.
Unless during the Obunghole years things have changed, it is impossible to get into the military being illiterate. You MUST take and pass the Armed Services Vocational Aptitude Battery (ASVAB) exam.