I just made a post about this in the "Thetford?" thread you might want to read. If you have a leaking seal between the top and bottom (bowl will still hold water) the Thetford will leak on the inside down into those cavities and over time, evaporation, refill, dry up they will become VERY stinky. The cause could be a bad seal, OR, a loose screw holding the top to the bottom. There are 2 at the back behind the bowl - one on each side if one or both are loose it can leak. Any way you slice it I would just buy a Dometic and replace it.
Trying to clean those cavities is NASTY and you have to pull the toilet to do it. If you have to pull it, just drop a Dometic right back down. I would warn you to be careful if you put the old Thetford in the Dometic box and then lift it over the edge of a dumpster.....those cavities are full of foul smelling, yukky stuff and it WILL run out of those cavities and right down the front of your shirt - no, I don't want to get into it...again!