Well, a Google search for your 325 system leads us to this info. There were actually choices of info, depending on whether your system's switch control has or does not have "in park light"
For does not have try this link:
For does have try this link:
Not sure it maters since your question seems to be about the valve location.
Page 29 shows the actual dump valve is on the "6 pack" valve control body. Where that is in your coach, I have no idea. But all the air bags air lines lead to it. Back track, or listen for it. Mine happens to be in the front of the coach just behind the front axle, not far from the dash control.