Can I deduct the cost of renting a wheelchair handicap accessible van to go to doctor appointments?

Yes. There are many medical related expenses that the IRS will allow. Publication 502 has this to say:

Transportation

You can include in medical expenses amounts paid for transportation primarily for, and essential to, medical care.

You can include:

• Bus, taxi, train, or plane fares or ambulance service,

• Transportation expenses of a parent who must go with a child who needs medical care,

• Transportation expenses of a nurse or other person who can give injections, medications, or other treatment required by a patient who is traveling to get medical care and is unable to travel alone, and

• Transportation expenses for regular visits to see a mentally ill dependent, if these visits are recommended as a part of treatment.

https://www.irs.gov/publications/p502#en_US_2017_publink1000179021