If a person today accepts and believes in what is required to believe in Islam, but dies while committing major sins such as zina, drugs or alcohol, are they doomed to Jahannamn forever? Would Allah never forgive them? Someone told me such a person is a munafiq and in jahannamn forever is that true?

No this is the aqeedah of the kharijites that major sins = forever in the hellfire. A person who has established tawheed who is upon major sins is still a Muslim and enters into jannah whether initially if Allah firgives him and wills it or he has hasanat that outweigh sins, or eventually after being punished.