I personally wouldn't be doing it this way as your actually asking the VSR to charge two additional batteries - which it probably isn't designed to do. Potential also being you'll just end up draining your start battery anyway. If you are going to drain a battery - you want it to be your second battery.
If you want it simple, run your cabling direct from the second battery to the towbar with an inline circuit breaker at the towbar end installed for safety. Worse case scenario here being you drain your second battery, start battery is fine though.
If you think you may want to add additional power options to the vehicle down the track, in the tub for example, then run cabling direct from second battery to a fuse box, then cabling to your anderson plug direct to the fuse box. I've done that by using one of these, https://www.whitworths.com.au/blue-sea- ... fuse-block
, with one of these being used as an earth connection point next to the fuse box as well https://www.whitworths.com.au/blue-sea- ... -plus-5-16
If you want to get fancy, I'd be installing a ctek or redarc dc-dc unit in your camper which then allows for solar connection for charging your camper's battery.
Otherwise you could place a relay in the the anderson plug cable run, with the relay connected to your car's ignition, thus anderson plug only gets power when ignition is on. Alternatively you could go old school, and just disconnect the anderson plug when you park the car.
Yes... I'll have another beer.