Not familar with the 255, but on my 283ts all 3 lines are at the right rear of the coach.
What you'll be looking for are 3 (usually) 1/2 inch lines exiting the floor of the coach. You should have a hot water, cold water, and a fresh water tank drain. From the factory the lines were usually capped with a twist on plastic cap, but a lot of us have installed a valve of some type as the caps get damaged/lost very easily.
From left to right, blue-cold water drain, red-hot water drain, blue-fresh water tank drain.
First are the hot and cold lines with valves I installed.
Second shows all three (fresh water drain has it's own internal valve)
Third shows location on coach.