DC31 or AGM?

Why not just get two indentical "middle of the road" crank/accessory combination batteries. Something in the middle.

Our car initially came with two identical batteries under the bonnet as standard. When we started setting up for camping and remote trips a friend, who owns a large battery shop, suggested that we fit two the same. We decided to give it a go (at a discount helps).

And to enable us to set up camp in the bush for more than two days without having to charge and running two fridges we fitted another third and a Redarc isolator between one (for cranking and standard car power) and the other two separated for 4WD and camping accessories and lighting etc.

We have three 105 AH Allrounders under the bonnet or our 100 series 4.2TD.

We have camped for three days and all car power and accessory power has performed without a hitch.

No complaints after six years. The one that did all the cranking broke after being shaken about on the Canning. Five minutes to swap it with one of the others and we were away.

Interested to see if anyone else does the same or similar?

My new van came with an 80Ah Delcor calcium battery. This was nowhere near a big enough capacity as I had upgraded to an upright compressor fridge in the van.

I bit the bullet and sold the Delcor and replaced it with two 105Ah Fullriver AGM batteries.
These have a great reputation and I know these are going to be looked after by the on-board dc-dc charger. They are currently $375, but can be bought on special for $349.

Not impressed with the Calcium battery. They need a higher charge rate of about 15.7 volts to obtain a full charge. Not many chargers will supply this higher requirement.

AGM's will fully charge with a 14.2 input voltage that most alternators will give, excepting some modern vehicles' tricky dicky lower output alternators. This is where the dc-dc charger improves the charging process so the best charging resume is obtained.
Obviously, voltage drop comes into play, especially when charging van battery banks, but a good quality dc-dc charger will boost the voltage if necessary to provide an optimum charging voltage with a multi-stage process, thus ensuring a long life from the battery investment.

We had AC Delco calciums in the first CT and had no trouble with them. Could take a bit of abuse. Charged fine off the alternator. $170 each for 96 aH. In the new unit there's Ozcharge AGMs that are a good deal more fussy. c. $375 for 110 amp hours, each.