Sara, since your batteries are in the back compartment, I'm going to modify my suggestion. Buy the Victron BP-65 and not the orange switch. Remove the black jumper wire from the plug in the photo of the Victron. Wire that to any switch mounted on the outside of your camper where you can reach it with the camper closed - this will have low voltage and current running through it. The switch commands the BP-65 to cut off or enable the battery. There is a very minor current drain of 6/10 of a milliamp in "cut-off" mode, which is negligible.
This requires only one additional battery cable, and low-voltage wiring for the switch. So, it's pretty simple, but I'd still suggest that you have someone with electrical competence do this. Use the diagram in
this manual, the switch is labeled "remote" and you will not need the program switch, the LED, or the buzzer.