It really varies depending on what model toilet and pop-up camper you have. My Flagstaff T21TBHW A-frame has a Thetford Model 230 series cassette toilet that has water supply permanently plumbed in, but does not have a water tank.
After spending time poking around the Thetford web site, I was able to find an installation manual, an operating manual, and a graphical parts list. In my case, the toilet enclosure hides all the workings, the only access is through the cassette outside door. The water line runs through there. Removing the cassette helps with access, it's almost impossible without removing the cassette.
I believe the toilet removal from the enclosure can only be done from behind the enclosure, but the installation manual and parts diagram should help you figure it out.
There are also YouTube videos, but be aware that the poster may have a different model Thetford than you do. A lot of New Zealand/Aussie videos about Thetfords, where other models are more popular. Thetfords are made in Holland, and are expensive to replace ($650+). I managed to find a company that sells just the cassette (I broke the door mechanism yanking the cassette out with the toilet base door open) and a new seat for $250+. Delivered in 2 days.
hope this helps
2019 Flagstaff T21TBHW A-frame
2008 Hyundai Entourage minivan
camping Colorado and adjacent states one weekend at a time