Keep in mind that when you test things as you typically would, Monaco wired a lot of things by breaking the ground to switch, rather than supplying 12 volts. I know the exhaust brake is wired this way.
If you say the piston is working, maybe a test drive with someone in the bedroom to watch the PacBrake. I videotaped mine to see that it wasn't working.
Maybe the customer isn't talking about the exhaust brake, but the trans is not retarding/downshifting like it should. As you know, two things happen when the exhaust brake is activated. The trans downshifts and the exhaust brake flapper closes.
Lastly, on the Monaco coaches there is what they call the "run bay" in an outside compartment under the driver's window. There is a large plastic box with all the fuses in it. Under that box are either two small boxes that say Monaco 1 and Monaco 2 or one large box that was used in newer years. Those boxes have all the relays and fuses for the exhaust brake. Also, there is an Allison box against the right wall as you look into that bay. If you take the cover off, there are 5 relays in there.
Don & Mary
2019 Newmar Dutch Star 4018
2019 Ford Raptor