I use this system,
https://www.amazon.com/KERUI-Securit...s%2C149&sr=8-4 you can buy different packages and ala carte parts to fit your needs. The siren works off a 12 volt wall adapter but you can cut the wire and wire it to your 12 volt RV battery. I have 2 sirens, 1 n the fridge compartment and the other one I kept as a plug in and I use it when I am at a motel so I'll hear it in my room. You can have both sirens sound at the same time. I wired up plugs so I can plug my sirens into the vehicle battery or the wall transformer as needed.
I used the little door sensors on the motorhome and when I travel in one of my trucks/cars I take the motion sensors with me. I have magnets glued onto them and I can put them inside the camper shell and under the hood, and another on the console. Some folks put them under the car to protect their catalytic converters, there's a video out there showing how to do that with this system.
I also rigged up a blinking red LED by the windshield that I can switch on and off. That alone will make a thief think twice.
The only drawback to this system is it does not notify you of an alarm event, just the siren but that's good enough for me.