I have a 2001 Monaco Dynasty 38 ft, with an ISC-350 that uses a PAC brake with only a foot pedal to activate the exhaust brake, which I assume would be very simlar to your coach.
About a year ago I had a problem with my PAC brake not activating when depressing the foot pedal. After checking the fuse which was OK, my next step was to test the Bosch style electrical relay that was located in the Front Electrical Run bay that is accesed through the same bay door, directly under the driver, that contains the PAC brake fuse that you checked. After I replaced the PAC brake relay, my exhaust brake system has functioned flawlessly ever since.
These are a very common, standard 12 vdc relay, (blade style, plug in relay) that are a small, black colored, square looking cube that can be purchased at NAPA stores or most any retail auto parts store. As a word of caution, make sure your N.O & N.C. pin-out configuration matches the one you purchase as there are a couple of variations. I would suggest replacing that relay before replacing any other expensive components. As you are likely aware, these systems are difficult to do much static tesing while parked in your driveway, as you almost have to test drive it to determine that the system is functioning properly.
As info, for the PAC brake to function, the exhasust brake system has to sense a 0-throttle position from the engine ECM before it will engage.
Hope this helps your efforts to correct your problem.