Most auto alarms can be adapted for use in an RV. Most of the Newell made during the 1990's had an alarm system built in. There is a panic switch in the bedroom and a switch inside and a key switch outside that when both turned on, will honk the electric horns if the door is opened. Other options include
http://desertsecurity.com/products and
http://flagshipsentry.com/rvsecurity.html.