I've been storing various TT and PUPs for over fifteen years and have never had a battery stolen. Looking around the storage facility the propane tanks would be more profitable to steal.
The solar is nice, but you have to analyze how much sun the battery box will receive each day. Your batteries will be between your TT and the propane tanks. With all that shade it doesn't seem like the trickle charge from the tiny solar panel will be enough to keep your battery charged.
Finally the battery box is over 8" wide. Measure the distance where your battery mounts, mine is less than 8" wide.
So skipping the solar for a moment there was a thread here from a couple years ago. Basically to deter theft someone made a cage from stainless steel that they could lock their battery into.
Even simpler is this