Pete....It's your parking brake alarm. Your parking brake, located in the panel to the left of your leg, has a brass air/electric valve screwed onto the bottom of the parking brake switch. The valve is about the size of your thumb. Remove the four screw cap covers and four screws that hold the side panel cover on. When you open it up, there will be a LOT of wires. The parking brake switch has a red and blue air line attached and electrical connector. You need to remove the screws that hold the parking brake switch to the panel, pull the whole assembly out and replace the valve. It's an easy repair, once the valve is out where you can get a wrench on it.
The company that makes the valve, Nason, makes the identical valve in multiple internal and electrical configurations. Don't waste your time trying to find one after market. Call Monaco parts, it's about $55.00 shipped to your house. I replaced two in 10 years.
The alarm only sounds when you put it into gear and then does so fro a few seconds up to a few minutes. Here is a test. Put the coach into gear, release the brake. If the alarm is sounding, engaged the parking brake, "pull up" and then "slam" down firmly. The jolt will usually make it stop.
If you send me a PM with your email address, I'll send you the photo and part number. I can't get it to post here.
Don & Mary
2019 Newmar Dutch Star 4018 - All Electric
2019 Ford Raptor Crew Cab