As it turns out... nobody was right.
So, I picked up the parts I listed above...
TUrns out -- the ball, catridge, shaft is not the correct one. There is a replacement available that has a plastic shaft... and a different part if you want the brass shaft.
If you get in touch with
http://www.marinesan.com - they are well versed in marine plumbing. I was in their store today.
The correct valve/shaft combo is a 385310954. It runs about $50 and fits our model 5148 toilets with the rear discharge.
Here's where we get wacky - the bowl seals
385316140 - Bowl Seal Kit -- is still a valid part number. Some of these, however - have an overflow hole. Some of them do not. Our toilets do NOT need to overflow hole. If you get one with the overflow hole, then when you lay down the rubber seal over the teflon seal, offset the holes. Easy workaround.
This is been a process for me... because I wanted that brass shaft. Dometic apparently likes to change part numbers around.