Here’s a link to Thetfords cassette toilet page
Lots of interesting pictures ….
Thetford also makes a marine model of portable toilet with a pump-out discharge port that could be used with a macerator to pump out the tank….
Sure you could cut a hole in the lower tank and glue a section of black plastic pipe and a valve on it ….but you’d have to do some research on the best glue to use for this sort of project.
But since you’re dealing with some pretty thin plastic on the holding tank portion of the toilet, every time you sit down on the toilet, you’d be putting stresses on the glue.