A wise man once said that the art of not being victim to some thief is to simply make yours more work than the next guy's. On which basis, even simple locking devices will probably do the trick, seeing as how most people that I've seen do not lock either propane tanks or batteries.
I confess I don't. But then my camper is stored at my house, and neither house or camper can be seen from any public right-of-way. I DO have a lock on the hitch, making it non-trivial to drag the whole thing away in the dark of night or when we're not home. And sometimes I will put that on when we go camping, but mostly not.
I have been in a campground just once when somebody came through raiding folks sites and "basement" storage bins in the wee hours. But the perp was looking for beer, and ignored tools, batteries, propane tanks, bicycles, lawn chairs, and such. We know this because the main thing missing was, well, beer, and one cooler of freshly caught fish, which we suspect was a big surprise when he got to the party.