how do i replace or test the flusher pump on caravan cassete toilet?

If its the model I think it is try this:
Take out the toilet roll dispenser.
The canister for the toilet roll holder is only held in by a large "O" ring. Grip the edge of it an pull it upwards.
You should now be looking at the pump submerged in the flush tank water.
The pump is held by one stainless steel self tapping screw.
Note: if the pump hasn't been used for a while you may find the plastic impellor shaft has become stuck. If so just give it a couple of turns manually with a soft (plastic) pointer to free it.

Cledlyn out of Wales
July 2009