The black plumbing is under the floor; unless someone has modified the trailer, all the fresh water stuff is above the floor aside from the fresh water tank.
There were a variety of layouts; ours is a rear bath. The copper tubes for the shower went under the shower pan; the bathroom was pop-riveted together polystyrene afaict. The water pipes were also intertwined w/ the furnace ducting along the side of the trailer.
I would NOT replace the plumbing unless it either leaks everywhere, or you have the trailer apart anyway. If the trailer is coming apart, changing out the plumbing is not difficult and costs are reasonable - perhaps two or three hundred bucks in parts.