I never had a problem with stolen propane tanks, but my battery was stolen. Take both home with you. Put the battery on a smart charger and keep an eye on the water levels. The cable that Iraqvet05 mentioned sounds like a good idea to deter theft if you don't want the hassle of removing the tanks. Battery should come home with you and be maintained.
I have never used my fresh water tank. I just hook up to the CG water, using a good pressure regulator and a inline water filter from Walmart. We usually buy one of those big water containers for coffee though.
Happy Camping!