Yes, this is correct. When calculating the non-taxable portion of your RMD on Form 8606, ALL of your IRA Accounts (Excluding Roth) are taken into account regardless of whether they are made up of Deductible or Non-Deductible Contributions.
For more information, you can click this link to see IRS Pub 590, Distributions from IRA's starting on Page 13: https://www.irs.gov/pub/irs-pdf/p590b.pdf.
