Motorhome Alarm Systems

Whilst my motorhome has an engine immobiliser it does not have an alarm. So far in our short history of owning a motorhome we have not actually left it anywhere other than a campsite. However if we want to visit places en route or even go shopping without one of us staying the van an alarm seems a sensible upgrade.Are there systems considered better than others? What are the likely costs? Are there any downsides to having an alarm?

Thanks

David