Deep cycle batteries

Most deep cycles accept charge at a slower rate & ideally should be charged at a slightly higher voltage as well making solar panels ideal.
Cheers Craig...............

"Wet cell" deep cycle batteries take longer to charge fully from a vehicle alternator. In fact, it may not be possible to get a 100% charge via the alternator in a dual battery setup.

AGM deep cycle batteries however, will charge much quicker and to 100% charge from a vehicle alternator.

Regardless of the type of battery though, a reasonable solar panel will have a beneficial effect on battery life.

If you take a 40 litre Engel as an example, the maximum draw from the battery is 2.7 amps (for the current model) and in practise will be somewhat less as the fridge cycles on and off. The actual amount of current draw will depend on ambient conditions and how high the thermostat is set but again, taking a 40 watt solar panel as an example, you can expect in the order of 2.5 amps in normal conditions so you will be putting back about as much as you take out (or more) for several hours per day.

Cloud cover, etc. may have an adverse effect on the amount of current you return to the battery and larger capacity fridges, with a subsequently higher current draw, will determine the need for a larger panel.

In my practical experience I have got a minimum of 3 days running the fridge of a 75 Ah+ AGM battery pack, supplemented by a 40 watt panel. At the end of this period the battery pack was still showing "charged" and I believe I may get much longer from my setup if I needed to, but I have no desire to "experiment" unnecessarily.

I have the backup of an auxiliary battery in the vehicle and the ability to also charge both the auxiliary battery and portable battery pack from the vehicle alternator.

This setup may not be the "optimum", but it works for me and I have an expansion path in adding another 40 watt solar panel if necessary, for a moderate cost outlay.

Hope this has given you some ideas.