Zig, battery charger and cables sizes

[AlexIRL]

Hi all,

I am renovating a camper and I would like some advice about the electrical system.
The camper had a Zig cf unit. So I am thinking of still using it.
However after reading a lot about the different systems I am a little confused about some aspects:
- 1st- Fridge connection. It is recommended to connect the fridge directly to the main battery, the Zig instructions recommends using a relay RM12 or RM14. However I did not find information about how to connect these relays to the alternator of my van (I am not quite familiar with the van wires). So I decided to use a sensitive Smartcom relay instead (more simple to install).

- 2nd -Battery charging: I read that the Zig is a trickle charger. I also read that this type of charging needs a lot time and it is not able to charge the battery completely. So once I have the Smartcom relay for the Fridge I thought of using it also to charge the battery a little faster or to complete a final charging step. This will have a switch to use only when required. No idea if this is right. I am also wondering if the fact that the battery is charging from the Zig and the Smartcom relay at the same time can be bad for the leisure battery.

-3rd- Cables sizes. This seems easy regarding the famous formula: Cable size=(Amp*Calbe length*0.017) / 0.3. However it s not clear to me what is the right size for the cable coming form the engine battery to the Zig and to the Smartcom relay as I do not how many amperes this cable will use.

I read that the battery should be connected using 10-16 sq mm using for example the Smartcom, obviously the Zig unit does not allows this cable size. So my question is what is the right cable size from the engine battery to the Zig unit if they are separated by 5m and what are the Amps that would support this cable.
Same question for the 30Amp Smartcom relay.