Slow Water Drainage

I have a Coachman with rearwashroom, the drainage from the Kitchen is fine but the drainage from the washroom is virtually non existant, we have to create a vacuum to empty the bowl. it will not drain on its own, I have tried Thetford Tank cleaner, Domestos, and a drain pump that puts a pressure of water to cleare any blockage, I've used rods, you name it I've used it, has anyone else experienced this.if you have and have cured it PLEASE TELL !!  It is very annoying as the bowl in the washroom is directly over the Wastemaster as the drain outlets are in the rear of our 'van.