According to Home of 12 Volt in Mt Barker, SA, you can back charge with your portable solar panel plugged into the 12 V point on the outside of your van. I had them install Anderson wiring into my van so now can plug panels into this.
Wiring is heavier duty.
This also solved my issue of isolating the car battery from the van and fridge when driving and stopping for short times.
Had Anderson system installed in the The Beast so now don't have to unplug the van during short stops.
Have been caught out once when I forgot to do so and flattened the car battery. Once bitten....
Was worth the investment. Upgraded van and had Anderson plugs/wiring installed again in new Panda. Battery charges when Travelling, runs fridge and completely isolated when ignition turned off.