I cannot suggest an alarm as I have not had to fit one but insurers will accept most makes and give you a discount. Do be aware though that alarms will run the battery down fairly quickly if the caravan is not connected to the mains, probably you would get no more than two weeks out of a charged battery. If it is in store or there is no mains connection then a good solar panel will be needed, but that needs a regulator to prevent overcharging..