wouldnt it be better to say "gdzie muszę spać?"
But that would be "where do I have to sleep". It seems rude.
If anything "gdzie mogę/ mógłbym spać?" - where can/may I sleep would be more polite.
Why we do not use: gdzie mogę spać? or gdzie ja będę spać?
Just because they are too far grammatically from the English sentence, even if they may sound better (at least if you omit 'ja' from the second sentence, or emphasize it).