I just replaced the seal on my Thetford Aqua Magic V toilet. To access the seal I removed the toilet and then I could see the seal and remove it, then install the new one. Actually I just removed the seal and cleaned it, then put it back in. I have heard of people who replaced the seal without removing the toilet???
When you disconnect the water to the toilet, some water will come out the pipe so keep a towel handy.
Is it messy? Yes! The area below the seal will be quite dirty. When you remove the toilet take it directly outside and use a hose to clean the mess out. Once it's cleaned out its not messy or difficult.
I used the old seal used to seal the toilet to the black tank pipe. It's not a standard household seal.