On my van I have 40mm PVC drainage pipes & attached to that is a 40mm flexible plastic hose to take it to the drainage grate.
Whenever the shower was used the water level in the shower base started to flood & build up.
After checking the PVC pipes to ensure they were all clear I added a small breather pipe to the top of the outlet fitting to prevent any air locks, but problem still persisted.
I then discovered the wife flushed tea leaves down the sink hole which travelled all the way to the end of the flexible pipe but settled at the end restricting the flow where the pipe goes up over cement lip at the drainage grate.
Now that we have changed the practice of tea leaf disposal the problem is solved and drains well.
There's a lot to be said for using tea bags when on the road.
Put wet rags over the inside plug holes & put the hose on full pressure into the end of the sullage hose or better still, the outlet pipes on the caravan.
I remember as a kid in the 50,s that my job after tea was to empty the teapot outside down the gully trap.To this day we have never put tea dreggs down our sink. My dad used to put Phenyl down the gully trap once a month.
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Westy. Some people I know are like slinkies. They look really funny when you push them downstairs !