I only have a single 12v, so it's easy enough to remove it when it's stored during the winter. Even when I had 2 12v's, I took them both out when it was stored. I leave them in during the camping season between trips and have been lucky so far I guess. It's parked on my side yard at home, behind a little picket fence, but visible from the street.
I've seen some pretty cool setups by some members that have put diamond plate boxes on their tongue with lockable lids.
I have a cable lock around my propane tanks, but it wouldn't take much to cut that.. ;) Just keeps them from walking away too easily is all.
Good luck!
Mitch