Sorry to hear about losing your batteries...
Not knowing your situation but alot of us folks that live in these high crime rate places have the batteries located in metal boxes with lever locks...
I tried to determine if your batteries were in front of the propane tanks or between the tanks and the trailer.
IMO the best spot for batteries in between the tanks and the trailer. This sort of hides the batteries from view but I guess everyone will know there will be batteries behind the tanks haha...
TORKLIFT makes a very nice slide on lid type metal box that is popular...
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They also make one that comes with a solar panel to trickle charge your batteries with
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I guess there is really not much you can do to prevent the batteries from being stolen. Make it harder to do is probably the best thing you can come up with...
I'm thinking about using one of those truck bed 48-inch side mount metal boxes when I upgrade my batteries here... I have tons of room to work with here haha...
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My batteries are still in the plastic boxes across the tongue of my POPUP trailer... I had four batteries here at one time but had one fail back in 2009.
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I like the idea of motion sensor lights where the trailer is parked...
They also go after the propane tanks around here... The drug guys use them for something they tell me...
I am fortunate here to be able to park my trailers here at the house in the back yard... My lights all come on when anyone walks up back there...
Roy Ken