Finally I have installed the pipe in front of the wheels, I have no place in the back of the trailer.
I had a remaining part in composite.
I cut two pieces in this one to fix the pipe under the trailer.
As it is installed in front of the
axle, I think of installing a kind of guard mud for the width of the caravan to protect better the pipe, against the objects which could pass under the caravan, as rocks, racoon, and more....