@Sonya You do not need to mention the refusal in the cover letter. On the application form you are asked if you applied for a visa before plus where and when biometrics were collected. Your information is on the Schengen VIS ( visa information system) shared by all Schengen states, your biometrics identify you and the immigration officer can see your immigration history. To have a chance of this visa being granted you need to show strong ties to your country of residence, which you obviously failed to show in previous applications.
@aprilleshea36 I repeat a Tunisian degree bachelors or masters are NOT recognised in the US or Europe. He is lucky to have a job due to high unemployment in Tunisia but even if it is an American company he will be paid local rates and not a US salary.
@aprilleshea36 Do not stay with his family for many different reasons. Different culture, different food, no privacy, security and by staying with his family you are openly stating you are his woman. Visit by all means, but stay in a hotel and as Terry said a decent hotel