Superdrain!

Caravan manufacturers don't understand hydraulics.
They use convoluted plastic pipe with quite a small bore and then make things worse by using even smaller connectors. The convolutions hold water back that stinks and the small bore ensures air locks and sinks that won't drain.
I can't do much about the piping inside but at least it is vertical and drains.
Outside it is now all 32mm plastic, the elbow is demountable for travel.