I'm not 100% sure just shutting off the battery switch will eliminate power to the landing gear.
I thought flipping the battery switch just stopped the trickle load of refrigerator controls, A/C controls, antenna boosters, and the like. But power was still available for the front jacks since no power is consumed until the landing gear switch is actually used.
Now, removing the battery completely might slow some thieves down just a little bit.
But, I think it is like being struck by lightning. How likely is it that a person would have their RV stolen a second time, especially a fifth wheel? Is this actually a common problem in the area?