My right leg (thigh) has been numb for months. What could be the cause?

Meralgia? If it involves the lateral aspect of the thigh and is not associated with any weakness, one possibility is meralgia paresthetica. This condition is due to compression of a nerve by the inguinal ligament, and can be relieved by weight loss. If the symptoms are persistent, medications for neuropathic pain or a local nerve block can be helpful. Meralgia can also be a first sign of diabetes.
Pinched nerve. Either in spine or near lumbar plexus nerves may be pinched. See a neurologist for some serious cause.