Betty, (Austin, TX)
Betty, it’s actually quite common for Thetford toilets to develop this problem over time. The situation is caused, in most cases, by foreign matter becoming trapped in the groove for the slide blade as your repair shop mentioned. It is possible they did not or could not remove all the debris inside the flushing mechanism groove. In severe cases the mechanism must be removed from the toilet and disassembled, cleaned, lubed and reassembled. Before reverting to that unpleasant extreme, fashion a hook out of a coat hanger or a bent screwdriver that will reach into the groove to scrape out the residue. Take special care not to damage the rubber seals. There is one located above the slide (see photo) and one below the slide blade. Do this repetitively until no more waste or tissue can be removed. Flush the toilet a few times, then lube the slide with a non-corrosive, petroleum-free lubricant like a Teflon lubricant or something similar. It takes a little bit of determination to remove all the debris. Perhaps your service facility did not exert enough patience the first time around.