First of all visit dentist  to know the exact cause of halitosis  .If you think it is due to dryness of mouth then a proper  investigation  of your medical history, your food habits is required. Im telling u few causes of dryness of mouth it might help you to rule out your problem:- *Mouth breathing *Tobacco smoking *Alcohol use *More use of caffeine like tea, coffee *Due to some medications *Radiation  to head and neck *salivary gland hypofunction. etc   Hope this will give you some idea.

Do Hexidin 0.2% mouthwash 10 ml for 1 minute,then spit it and don't intake anything for 30 minutes. Use twice a day for 2 week. Brush two times a day with bass method(ref youtube). Consult with a dentist as you may need scaling.