The only place we had a propane tank stolen was in the locked and videoed storage yard. (we found out after the fact we were parked in a video blind spot). The thief stole both full tanks and I did not notice it until we were setting up camp. He replaced them with empty tanks. Later found out it was the live in facility attendant who had his own trailer out back, and he helped himself to free propane tanks. Now the tanks are secured with a cable, and a second cable through the plastic cover.
The batteries are a real hassle to get to anyway, in the narrow space between the tanks and front of the rig. Now they are chained and padlocked, then strapped down, and inside a box. I curse every time I have to access them for maintenance.