I'm getting ready to donate a $25,000 custom power wheelchair, but the recipient won't assign a valuation for tax deduction purposes. How should I proceed to value?

For any item more than $5000, you need a signed appraisal by a qualified appraiser. I don't know who that would be in your case, someone who is qualified to appraise the value of a used speciality item like that. Perhaps a dealer in medical equipment.

You will need to complete a form 8232 and both the appraiser and a financially responsible official from the charity must sign it (the charity is not attesting to value, but to receiving the item).

https://www.irs.gov/forms-pubs/about-form-8283

You can e-file your return but you will required to mail in the signed original 8232 as backup after you file, turbotax should prompt you to do that and to print a cover page.  Keep a copy for yourself, of course.