There are two "bicycle" type brake cables that go from the foot pedals to the valve assembly. One opens the drop door and the other operates the water valve. Eventually the cables rust through and break, thus no flushing.
I've replaced both of our cables using parts from a bike store since both have rusted through at different times. Total parts cost $12. New ones are already rusting, so I definitely will be upgrading to Dometic next time the Thetford fails.