I have mild to moderate sleep apnea (AHI= 10, desat to 88%) and mild hypothyroid (TSH 5.01). Which is likely causing my fatigue? Which should treat?

Both. It would be easier to treat hypothyroidism as it requires only taking a pill everyday. Once the treatment gets going, try to lose weight and have the sleep apnea re-evaluated after losing 5-10% of body weight. You need a prescription from your doctor. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Drink enough water daily, so that your urine is mostly colorless. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Practice safe sex, if you have sex.