Welcome to the forum!
If your intention is to only power the fridge while you're driving, just use a relay that's activated by the ignition then proceed with power to the rear as described below.
If you want the fridge independently powered so that you can stop for lunch without it losing power, you need to isolate the second battery (Redarc sell isolators that work, as do Projecta). The isolator will protect the starter battery from overdischarging and will allow the auxiliary battery to be charged once the starter battery has been charged sufficiently.
In BOTH cases, you want heavy cabling running to the rear so that you don't lose a lot of voltage. Waeco fridges are sensitive to low voltage - that's normally a good thing, because it saves your battery - but a long thin cable will present an
apparent low voltage on the end of the cable causing the fridge to trip. When I say "heavy", I don't mean "20A" cable you can buy in K-Mart. I'm talking about 4Ga cable like
this from Jaycar - it's a lot more expensive per metre, but the overall cost isn't too bad when you consider that the end result is a far, far superior job.
You would do the same kind of cable for the Anderson plug in the rear (for the same reason). You'd only have to run ONE black (negative) cable down - it will cope with the trailer load and the fridge in the tub quite easily and the fact that you're not using one or the other of the red cables won't make any difference.
Hope that helps a bit!