The waste tank drains out via the (tiny) pipe outlet at the bottom, via a valve that is opened & closed by the red lever in the cupboard next to the fridge. There is also an overflow pipe which comes out of the top of the tank. Our tank was blocked at the main outlet.
To clear the blockage, you need to remove the round panel in the side of the fridge by the open/shut drain valve red lever, so that you can see the overflow pipe (vertical). This overflow pipe needs to be detached at the bottom - there is a jubilee clip to undo. We stopped up the bottom end of the overflow pipe with a bung, and put a finger over the bottom socket end. This is needed so that when you put water or air up the external drainage pipe, it goes into the tank at the bottom rather than via the overflow pipe at the top (where it isn't blocked). The valve should be in the open position.
We then pumped air up the external drainage pipe using the pump from our airbeam awning. Bingo! loud gurgling noise, and (quickly removing the pump), the water then gushed out of the drainage pipe. Result! We then flushed the tank through with a lot of clean water via the sink plughole. In fact I then put some tank cleaner in and some more water, and drove around with it sloshing around on our way to a campsite, where we emptied the tank fully.
Very happy that the tank is now empty and should not be smelly any more.
I've spelled all this out because it took me a hell of a time to find the relevant posts and actually get the thing working properly - understanding the set-up of the tank is key. Thanks to everyone who put useful advice on here.