So we would boon dock quite a bit and leave the trailer in the bush during the week un-attended.
With our old trailer I would just disconnect and store the batterys for safe keeping but two big deep cycles became a chore to handle it became a project to secure them on the new trailer.
I fabbed up a checker plate aluminum cowel to slide over the existing battery tray bolted to the angle iron on the four corners , the battery's are not removable without taking off the cover.
I then put a master lock cable lock through the battery cover(drilled holes) over top of the battery tray in between the battery's and now the cover cannot be removed even if you unbolt it.
As always nothing is theft proof if they are determined but this is no easy fast removal , thankfully it only has to be done a couple of times a season lol.