Thanks to Bill Atkins (Msg #105118) regarding dometic toilet seal replacement. His in depth post coupled with the "fix it" video (see link) persuaded me to successfully tackle this as a DIY'er.
Mission accomplished!
https://youtu.be/vi3TRivsNlQ
(I plan to use Thetford's toilet seal conditioner in the bowl prior to winter storage instead of RV antifreeze. The old seal was about 9 years old before it stopped holding water. The use of RV antifreeze may have accelerated it's demise.)