It would be best if you could swap the PROPANE tank to the front of the tongue and mount a good diamondplate lockable tool box between the propane tank and the trailer front wall. The propane tank would make your battery install more out of sight possibly.
I have seen several battery installs using these type of lockable truck tool boxes...
Sample photo from google images
This is a 48-inch truck side mount tool box which probably needs around 14-inches of clearance between the propane tank and the front trailer wall. Be sure to size up the area pretty good as the slope on some trailer fronts might prevent you from mounting here and be able to open the toolbox lid to gain access to the batteries. You may have your spare tire mounted on the bottom of the tongue also to deal with. Some spares are mounted on the rear of your trailer.
You can install up to 4EA GC2 6V Golf Cart Batteries in this lockable metal box. Some folks just mount two batteries and use the remaining room for storage of other outside items. Go to local hardware stores-Northern Tools-autoparts places and look at the available diamondplate type lockable boxes available. There are several photos for this install on google images. Search google for GILLDAWG79 and when it loads up select IMAGES from the left hand screen.
You can't totally make everything safe from thief but you can slow them down and maybe they will pass yours up.
My trailers are parked here at my house and i have motion detector lights on them at night.
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