I have noticed that when I drain the water from my waste tank it drains very quickly when the tap is first opened, then slows down to a trickle which could last for about 10 -15 minutes. This makes me think there is some sort of blockage. I am going to give it a good inspection when I get home but in the meantime am going to try a quick fix. I’m going to drain as much as possible then empty my hot boiler plus kettle and saucepan of boiling water into it along with a couple of washing up tablets. We will be doing a couple of hundred miles tomorrow so I’m hoping it might solve it.