Battery Box setup

Andersen plugs lose bugger all power. The correct lead, correctly joined into a couple of plugs will work fine. I drive company utes and they all have temporary dual battery systems in them that we make up out of low voltage garden light wiring, andersen plugs and a BEP VSR cable tied onto the earth lead of the main battery. This system works perfectly well back to the trailers we power through them.
LV garden light wiring is actually pretty heavy (8 odd mm?) but well priced from Cetnaj. It is getting towards the upper end of what you can physically fit into the crimp connection of an andersen plug.
I'll probably get told I'm crazy, but we've had these harnesses so long now, they're on their 3rd and 4th vehicles and still going strong.

Voltage loss is a very real problem and yes, you need heavy duty cable to counteract it.
Not the cheapest option, but I can recommend the Sidewinder Flyer as the most practical solution.
Not only does it come with the correct cable and Anderson connectors, but has a built-in dual battery isolator to protect the starting battery and fuses to complete the installation.

Sidewinder Flyer

Derek Bester of Sidewinder.com is an advertiser on this site and has solutions for every purpose.

If you cannot justify the initial outlay of this system, take a look at the wiring kits available separately. Down the bottom of the page on the link below is a Ute Kit.

Wiring kits

I did your setup with too lighter cable and I did not get sufficient charge to second battery to run fridge properly on an extended trip and killed my battery
I rewired with 2 B&S cable and all good as a base system ( I believe 4 B&S is sufficient but 2 B&S was cheaper to buy by the metre) , I have Anderson plug in mine too
Next step is battery type and dc-dc charger if you have a later model vehicle to get maximum capacity / life out of your battery

Voltage drop in both cable and connectors is avery real and calculatable thing.

Both are frequently under and over estimated by people.

You can actually charge a battery on quite thin cable, but it will slow the charge down.

Add a load to that system and the voltage drop will increase and possibly to a point that the battery will not charge at all.

All connectors have a finite resistance and thus present a finite amount of voltage drop, BUT if they are clean and in good condition this should be next to negligable

A SB50 anderson plug is specified as having a contact resistance of 200micro ohms per contact.

At specified plug capacity 50 amps
V = IR
V= 50 x .0002
V=.01 volts per contact, two in the plug .02 Volts voltage drop per plug

the same working for the SB120 rated at 190 amps works out to about .05 Volts at capacity

If the contacts are clean.....andersons are a self cleaning contact, they scrub each other every time you plug and unplug.

SO...hardly wth worrying about

cheers