Not sure why Jim’s coach has been set up differently, but we’re glad you've discovered it’s best to check the obvious first, Bill.
Flipping on the brake switch activates the valve that restricts exhaust gas flow, occluding the drivetrain with backpressure, and restricting forward momentum, saving wear on your wheel brakes, and is especially helpful descending mountain grades. In that instance your cruise control has less usefulness since it’s not good for inhibiting gravity’s effect on downhill motion. I think the cruise is (usually) cut off when the exhaust brake switch is flipped to on, although it may not be until the valve actually activates when letting off the throttle... your foot or the cruise. It’s been awhile since I drove. Others here may clarify that point. A moment later when the valve actually closes, a dash lamp appears saying it’s working. Unlike its companion brake switch “On” light, the separate brake actuated lamp (and the brake itself) only comes on when you let off the throttle. Hit the “gas” again and the brake valve opens and it’s ”actuated” lamp goes out.
It’s possible that lamp never activated to let you know because the valve is stuck open. But the switch being “On” likely cut power to the cruise control anyway. You should have seen a dash lamp that indicated the switch was on, even while parked and idling. All that said, your coach is a different year and model than mine, and could be configured differently in these regards.
I just assumed from Jim’s comment that ‘03 Contessas were different than most of ours with the smaller engines that don’t have compression brakes, but that come with exhaust brakes instead. Commonly called “PAC-brakes”, many like on our Monterey are actually made by Jake-brake, the namesake company for compression brakes.
I’d suggest you pop the trapdoor in the floor of your wardrobe and/or bedroom floor so you can view the top of your engine. I don’t know your Cummins, but on our CAT the exhaust manifold/turbocharger is seen along the engine’s street side. The exhaust brake is built into it and is easily accessible through that hatch. Check that the flapper valve moves easily. But regardless, get some lube and use it on the unit’s linkages as per the video available on YouTube, “PRXB Maintenance with Pacbrake’s SuperLube”. This product won’t burn off like others might, so don’t skimp and try to use another oil.
https://pacbrake.com/product/c18037-superlube/The valve can seize up and not operate properly when you need it going down mountain grades.
Go to the Home tab above, then the Private - BAC Members Only section, “Link to Coach Assist“, and follow links to any owners manuals relevant to your Contessa. If none, check the one for my 2006 Monterey where page 293 may prove helpful.
Joel