Update: I've only had a couple of hours to work on the trailer since last time, but I was able to r/r both dump valves. Time will tell if it's a good job, but here's what I did:
I'm using the back half of the old valve, which is glued permanently to the 3" black sewage pipe. My understanding is that if you purchase a gasket kit for these Thetford valves, you won't get the little grommet that seals the plunger on the side. That's why I got whole replacement valve, dissassembled it, and migrated all the new parts onto the old back half/sewage pipe.
Be careful with the steel rod coming in/out of the little hole with a fresh grommet. It's a tight fit, so I worked it slowly. But once the plunger's done, the rest is just matching up the new half to the old one, and make sure the gasket lines up with no creases.
Out with the old, in with the new/old. The valve on the left is done, the one on the right is about to be put together.
And if you could please tilt your head to the left, ladies and gentlemen...
All that's left is to install the mounting collars on the Black/Gray tank, and screw the valves/pipes onto them. I'm planning on using a 1/4" bead of plumber's putty around the collar as it slides into the tank. If you can think of a better way to seal it, by all means let me know. IMO this is the weakest link in all of the system as far as leak potential, so I want to get the assembly just *click, click of tongue* perfectly right.
And that will be the next photo installment. Hope this helps some other noob in need.