I replaced the valves on my '77 Safari rear bath this year.
The valves are available from Vintage Trailer Supply but I chose to go with the Valterra valves as they are easier to find and cheaper than the original Thetford.
The valves on the Safari are glued to the ABS drain pipe so I replaced all the plumbing from the tanks to the hose hook up. I used the Valterra flange at the hose hookup so I no longer need the adapter to attach my modern waste hose to my vintage trailer.
I used 1/4" threaded rod with joiners to extend the valve handles to their original locations.
I also started by towing the trailer to the local municipal dump station and flushing the tanks really well. I filled the tanks with water and a bag of ice and towed to the dump station and dumped, then filled the tank 1/2 full with fresh water and drove around to aggetate then dumped again. then refilled with water twice more time and dumped.
1977 Safari Land Yacht
2005 Toyota Tundra SR5
2010 Ford Flex Ecoboost