Following reading this blog about caravan grey water waste ( https://caravanchronicles.com/2013/05/26/connecting-your-drainage-on-serviced-pitches/ ) I purchased all the parts needed and have been very happy with using it. However, I have found that, if unsupported, the piping tends to sag and then the water doesn't drain well.
So I designed a little waste-pipe-support, and not being very imaginative, I took inspiration from the alphabet.
From just this one design, I printed off about 8 copies using the same model, but in Cura I adjusted only the height of each one to range evenly from about 80mm high, to 180mm high.
They can stack for saving a bit of space when packed up
These completely solved the draining issue.
I printed them in PETG for better weather/UV resistance
You need to rotate it so that it prints on it's side (looking down at the print bed it would look like a upside-down "V")
It may be also wise to add a brim or raft when slicing to aid bed adhesion while printing