"madiline"....This is a really common issue with Monaco's. Typically, the alarm goes off when you first put it in gear and drive away. Occasionally, they will go off while driving. If yours is going off when you first put it in gear and release the brake, pull the brake back on and then slam it down. If this shuts the alarm off, it's the parking brake switch that has gone bad. The switch is made by a company called Naston. They're suppose to last forever, but obviously don't. The switch is attached to the underside of your parking brake, inside that panel by your left leg. It's a brass switch with air connections and wires that plug in. It's about the size of your thumb.
You should only buy it from Monaco parts because they make about 20 different models of it. It's about $50.00 shipped from Monaco.
The only other thing that can make a similar alarm, could be your jacks. The tank for the jacks is in the louvered bay in front of the left rear tire. Lower the front jack, 6" with the ignition switch on. The alarm should sound. Go to the tank and have someone listen for the alarm. Remove the cap in the center of the tank and fill until the alarm goes off....that's it. Raise the front jack.....if that was your issue, it should be solved.