It's a fifth wheel so no battery boxes on tongue.
I would use some hefty hold down bolts with self locking nuts, so that you have to use a wrench the entire way to get them off. Yeah, my 20V angle grinder would make quick work of those hold downs, but it's loud and might damage the battery. Not much that you can do to prevent theft, just try to discourage it as much as is reasonable. I suppose you could get a Mosler safe big enough to hold the batteries and have it welded to the frame, but that would just be ridiculous. And someone might just see that as a challenge.
