I have a Thetford Aquamagic (5 I Think). I had to replace the seal last year. It came in a package with the flange seal as well, paid less than $10 for both seals.
I had to take the toilet off, turn over and open the blade. The old seal pulled out and new one back in, reinstall toilet with new flange seal, hook up water and good to go.
I was amazed at how hard that rubber seal had become over 10 years. We had a different problem caused by the seal, we did have a minor problem holding water, but the big problems was the blade not closing after flushing. When flushing, the blade slid past the edge of the seal and then would not close be the rubber seal had became so hard.
I was also thinking a new toilet for $200-300, but the $10 seal and only a little extra work solved the problem.
Ken