B-D RV Battery Lock:
http://www.b-drvbatterylock.com/
Even though I/we have never ever had an issue with a battery "walking away" in the 15+ years that we have been trailer camping I have been intrigued whenever I would see this simple battery locking system either in person or on camping forums.
A recent battery theft that Serge experienced (
) had me finally pulling the trigger.
So I called and ordered one on Monday morning and it was on my front porch yesterday (Thursday) early afternoon. 
Grand total was $32.95.
Luckily the 1575's spare tire is mounted under the trailer body & not under the tongue area. Also, the LP tank is far enough away from the battery tray that this is also not an issue.
*Both of these issues (spare tire & LP tank(s) location) cause difficulties apparently on all of the other Lance TT models besides the 1575 (& 1274).
Anyway, it is a simple, quick, but secure lockable bracket that fits easily around basic/standard group 24 & group 27 plastic battery boxes.
*It might fit around other boxes such as a group 31 box, but I have not tried it.
*They also sell a 6-volt battery box bracket as well.
I use a 4-digit combination padlock which uses the same combination as the locks on the coupler, bike rack, & side-yard trailer storage gate.
I fully realize that this will not stop a jerk from taking our battery while we are away from camp, but this will hopefully help him move along in search of an easier victim/target.
I didn't bother to load the battery in the box or hook up the battery cables, Victron battery monitor shunt, etc... for the pictures. I also didn't bother bolting the box to the tray which I normally do (you can see the 2 bolt heads in the first picture below).
A few pictures below with a group 27 box (which in our case holds a group 29 battery) -
