Distributions from a Roth 401(k) are a proportionate mix of basis and gains, so your RMD from the account will consist partly of gains unless the value of your Roth 401(k) has dropped below your contribution basis due to investment losses. Because you have not met the 5-year period for qualified distributions from the Roth 401(k), the portion of the distribution consisting of gains will be taxable.
Because you are over age 59½, there is no statutory or regulatory limitation on doing a rollover from the Roth 401(k) to a Roth IRA except that the RMD for the year for the Roth 401(k) must be satisfied first. It's unlikely that your Roth 401(k) plan agreement prohibits such rollovers, so you should then be able to roll the Roth 401(k) over to a Roth IRA. Because your Roth 401(k) is not yet qualified, the contribution basis in the Roth 401(k) will become contribution basis in your Roth IRA and the gains will become gains in your Roth IRA. If your Roth IRA is qualified (it's been at least 5 years since the beginning of the year for which you first made a Roth IRA contribution), any amount then distributed from the Roth IRA will be tax free. You can continue to make Roth 401(k) contributions, but each time you do a rollover from the Roth 401(k) to the Roth IRA you'll need to satisfy the year's Roth 401(k) RMD first.
