In my opinion, dual battery systems should be designed around the specific needs of the owner and not generalised as only one way to go and that is with a deep cycle battery setup. I have a Ford Ranger that I use as general adventure and tow vehicle. I have fitted a normal lead cell battery, 180Ah Truck battery, connected via a NL relay system.
This setup works 100% for me in a off-road, camping without power situation, as I cash in on the ability of the second battery to be recharged within a 30 min game drive. This supplies power for two fridges (80 l NL in trailer + 40 l Engel in my truck) set 3 and -3 degrees. The one used as a fridge is switched off at night. An inverter is also used at night to power my C-pap machine while I sleep. I have determined that my power needs would not exceed 30Amps per night and rationed myself to just that.
During a recent 5 week trip, circling the Okavango delta and at one stage camping for 8 consecutive nights in Botswana Parks, wihout electricity, I have never depleted the second battery below 80% of its capacity. The freezer in the trailer was disconnected during game drives and only the one fridge was powered. When not travelling, I would run both freezers for at least 6 hours before going on a game drive again.
My point is, deep cycle batteries cannot be charged in a short period of time, but large normal starting batteries can. Even if need to be, quickly be recharged by solar panel, in a fraction of the time a deep cycle battery.
This works for a situation where one can take a daily game drive and are on the roll most of the time and certainly works for me. ( The cost of such a large Truck/Tractor battery is approx R2500)
My 2c