If I convert 50K (which covers my RMD from my IRA to a Roth why does the 8606 only show the difference between the total amount converted and the RMD?

One thing.....you can't covert or rollover the RMD.

No part of a RMD is eligible to be rolled over.  Only that part of a distribution that *exceeds* the RMD amount can be rolled over or converted to a Roth.

Any part of a RMD that was rolled to another retirement account is an excess contribution to that account subject to a 6% penalty that repeats each year until removed.

TurboTax follows that rule and will not allow any distribution that is entered as a RMD to be rolled over or converted.

Any portion of the distribution that was RMD that was deposited into the Roth IRA is an excess contribution to the Roth IRA to the extent that it exceeds the amount that you are eligible to contribute to the Roth IRA as a new, regular contribution.