It is not clear that Thetford is actually still making the recirculating toilet. We have been told that they are not. You might be able to find one still in stock at an RV dealer or online RV parts store, but I would advise you to have the new one in hand before you start the project.
If you are talking about removing the Thetford recirculating toilet and replacing it with the same toilet, brand-new, it won't be too hard. You might start with this post in the TM Technical Library for an introduction to the toilet.
https://www.trailmanorowners.com/for...ead.php?t=2354
Then go to this article, also in the Tech Library for some info about removing the toilet from the floor.
https://www.trailmanorowners.com/for...ead.php?t=5889
Once you have removed the old one, installing the new one should be just the opposite sequence of steps. Be sure to use a new foam donut. Be aware that this donut is a different size from the ones you find in hardware stores, which are intended for home plumbing. You would buy the one you need from an RV dealer's service counter or the like. They are not rare or expensive, but you want to be sure it will fit.
Bill