I like the idea of placing the batteries in a metal lock box that has the 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 battery 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.
I use a long shank pad lock through the twist handle of my two propane tanks
All PHOTOs from GOOGLE IMAGEs
This will not keep your tanks totally safe from thief but will slow them down a minute or two maybe...
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 outside lights when parked.
Lots of info on this subject
Roy ken
My Posts are IMHO based on my experiences - Words in CAPS does not mean I am shouting
Roy - Carolyn
RETIRED DOAF/DON/DOD/CONTR RADIO TECH (42yrs)
K9PHT (Since 1957) 146.52M
2010 F150, 5.4,3:73 Gears,SCab
2008 Starcraft 14RT EU2000i GEN
2005 Flagstaff 8528RESS