My unit has the regular rocker on/off switch for turning electric water element on, and another for turning the propane burner for water heater on. If off, no electricity or gas will go to the water heater. The electric water heater element also has its own AC breaker in the camper's AC control panel. This breaker can be switched off as an additional fail-safe so you don't accidentally turn it on. Note that in my camper, the rocker switch will still illuminate if switched on for electric water heater even if the AC breaker is off. But the electric element is not receiving power in this instance. I'm not sure of an additional way of turning off propane. But the control panel of my water heater does have a fuse that can be removed when accessing it from the outside door. Removing that fuse will result in the heater not turning on with either electric or propane.
Found this image from another thread that shows the fuse I'm referring to. Underneath the flat wiring harness with various colored wires there is a gray 2A fuse (note that picture is rotated 90 degrees CCW so the fuse appears on the right of the harness). Pull that one, heater should be inoperable without it.