Look at a no-contract security system. I have a Fortress system in my home.
https://www.fortresssecuritystore.com/
There are other systems out there so look online for the best deal.
There is an optional app that sends you a notification on your cellphone if someone trips the alarm. If you connect it to their monitoring system through their cellphone service, it becomes expensive. If you have wifi on your RV, you can skip their monthly fee and get the notification on your phone for free. Unless you keep it near an access point, this won't work. Of course, their cellphone based monitoring system requires a cell signal or it won't work. Without connectivity, it just sounds a very loud siren.
Sensors include motion detectors, fire/CO2 alarm, window and door alarms.
A notice that you have cameras in use if you actually don't can expose you to liability problems if someone thinks you have a camera and runs over to your site expecting to have a record of an assault. This sounds odd but it has happened. You can install a camera for short money but a camera won't stop someone and the video may not have enough information to identify the intruder.