Can I deduct the cost of a mattress that was bought due to a medical condition ( Rheumatoid Arthritis)? If so, would I include it under medical expenses?

Yes, you can deduct the cost of the m attresses and boards bought specifically to alleviate an arthritic condition.

You will claim the mattress cost under medical expenses on Schedule A.

See the link below for list of expenses eligible as "Medical Expenses"

https://turbotax.intuit.com/tax-tips/health-care/medical-expenses-checklist/L6MkxxlyW