What can cause moderate to severe pain in the jaw, ear, temples, and neck? My dentist doesn't think that its a tooth, even though I recently had a root canal.

Head pain. The symptoms you are describing could be related to a sinus infection which could reffer pain to the area's you describe. So you might see your general physician or endodontist to be evaluated.
TMJ Disorders. Your describing symptoms of TMJ disorders . Please get checked by a dentist with good training and knowledge of TMJ disorders.
Possibly TMJ. If the pain does not feel localized to the tooth and there is no swelling, it may not be anything related to the root canal. Pain that extends to the ear, temples, and neck can sometimes indicated TMJ syndrome.
Lots of reasons. Could be from sinus infection, ear infection, trauma, neurologic disorder, temporomandibular joint or mho-facial pain dysfunction disorders, infected tooth/teeth, infected gums, infected tonsils/adenoids. In other words you really need to be examined. Start with both your family dentist and your general physician. They may rx or refer you on to specialist.
Second opinion? I would possibly consider getting a second opinion if your dentist insists that you do not have a tooth problem.