ESP GUY......your transmission is not the issue. Monaco had a very common problem with the air brake buzzer. On the panel near your left leg, while sitting in the driver's seat, you have the parking brake switch. It's bolted to the panel with three small screws. You also have a large side cover that your leg rests against. If you remove this panel, it will expose a maze of wiring. You'll also find two 3/8 air hoses (I believe red and blue) behind that panel that attach to the bottom of the brake switch. They actually attach to a small brass valve, about the the size of your thumb. It also has wires attached. This is the air brake sensor. I replaced three of them over ten years on my 2005 Diplomat. Monaco still sells this part via their parts store. It's about $50.00 shipped to your house. ONLY Monaco sells the appropriate one for your specific coach.
Here is a test that will confirm yours is bad. Release the parking brake and put the coach in gear. If it's buzzing/beeping put your foot on the brake, pull the parking brake handle up and then slam it down hard with your palm. This typically makes the sensor quick buzzing. Usually, driving for a few seconds will make it shut off. It can also buzz unexpectedly while driving, for a few seconds, and then shut off.
Don & Mary
2014 Newmar Dutch Star - All Electric - 450 ISL
2016 GMC Canyon 4WD Crew Cab