Most likely to be a jammed or knackered pump.
First to test for current at the pump turn on a 12volt light in the toilet cubicle press the flush button if the light dims you have current at the pump.
If the C2 bench type with toilet roll holder to the right facing the toilet. Pull out the toilet roll pull out the cup it sat in( its a loose fit no screws) the may need gentle prising around the rim (vasalene the edge edge before refitting so its easier next time.
look down past flush pipe you will see pump in a plastic cage. One cross head screw holds the cage down remove it lift that screw and lift pump clear if necessary pull off the flush tube. Turn the pump over and turn the impeller at the bottom using thin screw driver until pump feels free. don't fit it back in to the cage refit flush tube drop pump in the water then try the flush If the fuse is ok and it still doesn't work then its likely the pump needs replacing.
Cut the existing pump wire about a foot from the existing pump. You have to watch the polarity the live side+ is marked with a tracer colour eg white wire with black tracer. and negative - single colour.
New pump will be same, colours may differ. Go to car spares store get cable heat shrink joint sleeves slide a sleeve on to each of the wires on new pump pair and bare ends of each wire twist pairs of ends together pm their pairs. slide sleeves over joints the hrat shrink with hair dryer. Test pump in water do not run it dry.
If its not this type www.thetford-europe.com service look for sanitation download pdf covers all models.
Bob U.K., August 2013
