Pictures say a thousand words.
Before and after pics.
Take notice that my trailer has a 2 7/8" lift kit on it. If yours doesn't have the lift, your grey water discharge pipe will need to be routed under the axle. There's plenty of room and slope.
You just need 7' of 1.5" ABS pipe
1, 90* (sweep) fitting.
1, 45* fitting
1, small bottle of ABS cement.
1, Double gate valve.
1-2 hours of free time.
Here is the double valve that I used:
https://www.amazon.com/dp/B0006MRRTE...fed_asin_title