Alarm? Hmmmm, I would venture to guess that the reason manufactures don't offer it as a option is that the majority of rv's, trailers, boats, etc are stored in storage facility's that are not located where people would actually hear a alarm. Of course someone might, but most likely a alarm would go unnoticed the majority of the time.
As far as in a camping situation I guess I camp in areas where I feel pretty safe. I do use a hitch lock, but if I needed more I personally wouldn't camp there.
I do think a service like, Lojack would be great for rv's, trailers, boats. But I think the owner could actually have that type of service installed if wanted. I have it in my truck so I can't see where it couldn't be installed in my trailer.
Hitch lock's, chock locks, are a great deterrent but , someone who really wants what you have locked up is gonna try and get it despite a lock.