You probably need to connect the brake controller trigger nearer the brake light switch, between the brake switch and wherever the exhaust brake circuit joins the brake light wiring. And you might need a diode downstream from it to prevent backfeed into the controller circuit when the EB comes on.
I gather you have attached the controller to the feed line for the trailer brake lights (the plug that was probably standard on your coach form the factory) and that comes on with the EB?
Have you asked Monaco/HR Customer Service about a good place to make this connection? They may already know the answer.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL