Thetford makes a good toilet. The Style II is a breeze to work on but as other have mentioned, it is probably easier to buy a whole new one. These toilets are not residential toilets and the parts are plastic. They only have so many cycles in them. I am of the opinion that after 300 nights (2 people) you should rebuild it or replace it to avoid problems. Of course more people would shorten that life cycle.
It took me an hour to remove, replace parts and reinstall. But the parts on Amazon where almost $75. You can get a new Style II for $160 on sale.
Good luck on what ever you choose to do.