Mine is a Freightliner chassis and that relay/fuse box is under a small cabinet on the floor in the center of the coach under the dash. It has a plastic lid on it. I can take a picture of it later if you like so you know what you are looking for.
I just installed a brake controller for hauling trailers on my rig it was pre-wired so I just needed to find the right harness I looked several times for it and could not find it (lots of wiring under the dash) I looked up the pig tail on ebay once I saw a picture of what the plug looked like I found it right away.
