I use a long shank pad lock through the twist handle of my two propane tanks

This will not keep your tanks totally safe from thief but will slow them down a minute or two maybe...
I like the idea of placing the batteries in a metal lock box that PADDLE LATCH W/KEY LOCK verses a MASTER lock in a hasp type hinge lock. With a metal box you also have to deal with proper venting and drainage of battery fluids and gas exhaust.

I would also mount the metal battery box on top of the trailer frame with the mounting round head stove type bolts coming through the trailer tongue and bolted down inside the metal battery box. All of this will not totally protect your batteries from thief but will slow them down considerably.

The thieves will be equipped with bolt cutters and portable power saws and can get your batteries and propane tanks in just a few seconds time.
My trailer is also equipped with motion sensors out side lights when parked.
Lots of info on this subject
Roy ken