Thanks for your replies. It's been a very taxing day. I did some additional research and this was the most informative resource that I found so far:
$1Under question 4, which concerns whether and when an employee should disclose their disability to their employer, it says: "Sometimes, the decision to disclose depends on whether an individual will need a reasonable accommodation to perform the job (for example, specialized equipment, **removal of a marginal function,** or another type of job restructuring). A person with a vision impairment, however, may request an accommodation after becoming an employee even if she did not do so when applying for the job or after receiving the job offer." I would consider my once-a-year need to drive a station vehicle a *marginal function*. Hopefully management will agree that the removal of this function won't cause the company an undue hardship.
Under question 10, which concerns what reasonable accommodations an employee with a disability may need, it says (among other things): "**a driver** or payment for the cost of transportation **to enable performance of essential functions**." And one thing my boss mentioned is the option of pulling someone from another department to drive me to a shoot if I need to go on one and the other member of my department is out. So if the company does deem the ability to go on shoots without my other department member an *essential function*, this seems like a reasonable accommodation that wouldn't cause undue hardship to the company.
So, armed with this knowledge from the Equal Employment Opportunity Commission, I think I have a pretty good case for keeping my job. But there's also an okay chance that I might not have to fight for it because the GM may just say it's fine and we'll work around my disability in this one small area of my job responsibilities. My boss could just be making too big a deal out of it at the moment because he's afraid that the GM will be unreasonable and doesn't want to get my hopes up. But now, no matter what happens, the way my boss has handled it has done irreparable harm to our mentor-mentee relationship, because I truly feel like he doesn't have my back right now and is more concerned about the bottom line, which hurts.